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(a) Introduction to communicable diseases

Communicable diseases are caused by pathogens e.g. bacteria, fungi, protists or viruses.

Health is the state of an organism's well-being - physical or mental, but ill health is where there is a problem including suffering from some disease.

The World Health Organisation defines heath as "a state of complete physical, mental and social well-being, and not merely the absence of disease or infirmity".

So, even if you are a very fit person, you are not necessarily healthy e.g. if you had mental health issues and/or lonely.

A disease is a medical condition where part of an organism (plant or animal) isn't functioning properly - in some way the organism is not as it should be!

The disease may take the form of cell damage to the host (plant or animal) which in some way impairs the healthy ('normal') structures or functions of the organism.

Most organisms, including ourselves, experience ill health at some point in their life.

If you have an increased chance of contracting a disease you are described as susceptible.

There are many causes of ill health in plants and animals e.g.

infection from a pathogen eg flue, malaria, salmonella,

mutation in an organism's genes (DNA) eg cancers,

an organism might suffer some deficiency eg lack of vitamins in human diet, lack of light on plant growth

an organism may experience mental or physical trauma triggered by some event eg depression, bereavement, serious accident,

the lifestyle of an organism can have consequences on your health eg links between: smoking and lung cancer, too much sugary/fatty food and obesity and/or diabetes,

All diseases show symptoms at some point in their development.

Symptoms are indications of disease in an organism - usually observable eg cough, rash, diarrhoea, leaf discolouration etc.

Sometimes symptoms do not show up immediately after infection - the virus or bacteria may multiply for days or weeks until sufficient of the pathogen is present to create visible symptoms.

After a pathogen has entered an organism (infection), there is a period of infection without symptoms called the incubation period and may last hours, days, weeks or months - which is a bit scary, because you can't apply medical treatment to a medical condition you don't know you've got!

It is in the incubation period that harmful toxins build up.

The more pathogen present (bacteria or virus) the more rapidly toxins build up and you then experience typical symptoms like headache, raised temperature, stomach discomfort - fever etc.

 

Diseases can be classified as communicable and non-communicable.

Communicable diseases that are spread between individual organisms - animals and people or person to person.

Because communicable diseases can spread between organisms (plants or animals), they are often described as contagious or infectious diseases.

They can be caused by bacteria, fungi, parasites or viruses.

Examples are diseases like malaria, tuberculosis or measles.

Non-communicable diseases cannot be transmitted between individual organisms e.g. cancer, diabetes, heart diseases (eg cardiovascular) or respiratory diseases of the lung.

See separate page on non-communicable diseases 

When you have one medical condition e.g. a communicable disease, you may be more susceptible to another disease.

If you are suffering from one disease, your bodies defences may be weakened by it making you more susceptible to another disease - a 'knock on' effect reducing your body's ability to fight off a second disease e.g.

People with problems with their immune system by which your body defends itself against infections, may be far more susceptible to other communicable diseases such as influenza. The body is less able to fight off the infection from particular pathogens like the flue virus.

Lifestyle choices and your personal situation

Eating a good balanced healthy diet helps maintain your body in good shape and your immune system to fight communicable disease infections and reduce the risk of contracting communicable diseases.

The poorer you diet, the weaker your immune system is, by not accessing the correct balance of nutrients, therefore you are more susceptible you are to infection by pathogens.

Access to your needs?

The risk of infection from a communicable disease increases if you have limited access to good healthcare systems and health education.

When you have access to a quality healthcare system, your medical condition is more likely to be diagnosed and receive appropriate treatment. In turn this also reduces the chance of you passing on the infection.

Education provides you with knowledge about how diseases are transmitted and help avoid infection in the first place - see examples down the page on HIV and safe sex practice.

Usually no problem in rich developed countries - your choices, BUT not so for people living in poorer underdeveloped countries.

e.g. to help prevent or reduce the risk of communicable diseases in the first place - an you afford to buy healthy food? Is 'healthy shopping' readily available?

If you have contracted a communicable disease, do you have access to appropriate medicines?

Do you have access to contraception e.g. condoms to prevent the transmission of sexually transmitted disease.

Mental health and stress

If can develop a mental health condition such as depression while enduring some physical health problem e.g. lack of mobility reducing your ability to participate fully in everyday life.

If you are constantly under mental stress e.g. 'high-powered' job or caring for a very sick relative, then your physical well-being can be affected - ulcers can develop or a mental health condition like extreme anxiety.

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(b) Which types of pathogens causes communicable diseases?

Communicable diseases are caused by pathogens - types of microorganisms that enter an organism e.g. your body, and cause disease.

Pathogens can infect and cause diseases in both plants and animals, AND can spread communicable diseases from one organism to another.

Types of pathogen that cause disease

Bacteria

Bacteria are very small cells, compared to your own body cells, which can rapidly reproduce by cell division in your body.

They make you feel ill by damaging your body's cells and tissues and producing toxins - poisons produced as a by-product of the bacteria's cell chemistry that can damage your cells and tissues.

You should note that many bacteria are harmless and some are useful and some essential for a healthy body. Bacterial biochemistry makes cheese and yoghurt. Bacteria break down waste and they are very involved in the digestion of food, absorption of essential nutrients and are much needed for a healthy gut. There are 300-500 types of bacteria in your gut that work for your benefit along with viruses and fungi.

Unfortunately some bacteria are harmful pathogens to both plants and animals.

Viruses  (See also Appendix on viruses)

Viruses are NOT cells, they are much smaller than bacteria and damage the cells in which they reproduce.

Viruses rapidly replicate by invading a cell and using the cell's genetic machinery to reproduce themselves i.e. copies of the original virus.

The virus 'invaded' cell then bursts releasing lots of new viruses to go and invade other cells.

The cell damage makes you feel ill as your body fights back to make as many good cells as it can, to replace those destroyed by the virus.

Protists

Protists are all eukaryotes, often single-celled and there are many types and sizes of them.

Some eukaryotic protists are parasites and live on or inside the 'host' organism causing some kind of damage.

They are usually transferred to an organism by a vector which isn't affected by the disease itself e.g. an insect carrying protist e.g. malaria is caused by an insect (e.g. mosquito) that carries the protist.

Fungi

Fungi can be single celled or others have a 'body' consisting of multi-celled thread-like structure called hyphae.

Hyphae can grow and penetrate human skin and the surface of plants causing damage.

Hyphae can produce spores that spread to other plants and animals.

Larger organisms - parasites - some nasty species out there!

Helminths are a type of parasitic worm that can get inside your body e.g. tapeworms, flukes, and roundworms. Helminthiasis, also known as worm infection, is any macroparasitic disease of humans and other animals in which a part of the body is infected with parasitic worms, known as helminths.

Soil-transmitted helminthiases are responsible for parasitic infections in as much as a quarter of the human population worldwide. They can cause damage to the intestine wall, inflammation, damage organs such as the skin, lungs and live and also cause neurological problems.

Trichinosis is a disease caused by trichinae helminth, typically from infected meat, characterized by digestive disturbance, fever, and muscular rigidity.

Ironically, it is thought that infection by trichinae may reduce the development of some autoimmune diseases like Crohn's disease.

Autoimmune diseases are diseases caused by the body's immune system treating its own cells as if they are foreign and attacking them - a sort of overreaction.

How do we become infected with a pathogen (microorganisms or viruses)?

There are many ways in which we, as humans, can become infected with a pathogen e.g.

Pathogen entering our body through the skin when bitten by an insect or using an infected needle.

Sexually transmitted disease, when the pathogen is transferred in sexual activity - through contact of infected reproductive organs.

Pathogens breathed in through nose or mouth - aerosol droplets from somebody coughing.

Pathogens taken in through the mouth contained in contaminated food or drink (polluted water).

A brief note on symptoms of human infection (just some preliminary ideas)

The activity of the pathogen produces characteristic symptoms which arise from typically two situations.

(a) Reproducing bacteria produce toxins that are harmful to tissues and makes us feel unwell.

(b) Viruses invade cells and reproduce inside them, damaging them and killing them - this puts your immune system and healthy cell reproduction under strain.

These lead to having e.g. a feverish high temperature, nausea, headaches and rashes on your skin.

A brief note on preventing the spread of communicable diseases (just some preliminary ideas)

Being hygienic in food preparation - washing hands, clean work surfaces, storing food correctly e.g. in fridge.

Covering your mouth when coughing or sneezing and into a tissue or handkerchief.

Washing your hands after using the toilet.

Vaccination programmes against a potential viral or bacterial infection.

Isolating infected individuals in the home or hospital.

Destroying pathogen vectors (see malaria and mosquito).

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(c) How are pathogens spread?

There many sorts of ways that pathogens can spread.

Air

Pathogens are carried in air currents and breathed in e.g. fungal spores.

Airborne pathogens are conveyed through the air in water droplets when we cough or sneeze e.g. the influenza virus giving us 'flue' or tuberculosis.

When people are crowded together, their close proximity allows an easy transfer of a communicable disease from one person to another e.g. coughing out a cold or flue virus!

Direct contact

Pathogens can be picked up by merely touching a contaminated surface including someone's skin e.g. athlete's foot is a fungus which makes skin itch and flake off. It is most commonly spreading by coming into contact with a surface an infected person has e.g. shower floors or towels.

Water

Dirty contaminated water is a common source of pathogens and should not be drank or bathed in. Cholera is a bacterial infection spread by drinking water contaminated with the diarrhoea of people already infected with the bacteria.

Body fluids

Pathogens like Ebola can be spread through contact with infected bodily fluids of another person.

These include like blood (drug users sharing needles), vomit, saliva, faeces, breast feeding milk (mother to child), and sexual activity (contact with semen). HIV is a good example.

Animal vectors

Some pathogens can be carried and transported by an animal organisms referred to as vectors.

The mosquito is an example of an animal vector which carries the protist pathogen that causes malaria.

Soil

See plant disease notes - pathogens in the soil

Food

Pathogens maybe present in food.

Contaminated food may contain the salmonella bacterium causing food poisoning.

The Helicobacter pylori bacterium that causes stomach ulcers maybe found in contaminated food or water.

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(d) Examples of bacterial pathogen communicable diseases

The disease, pathogen, symptoms-effects, means of spread and how to reduce or prevent transmission are all described in the following examples.

Salmonella is a type of bacteria that causes food poisoning e.g. from contaminated food.

The symptoms experienced by infected people include diarrhoea, fever, stomach cramps and vomiting.

These very unpleasant effects are caused by toxins produced by the salmonella bacteria.

Salmonella food poisoning is often contracted from animals like chickens catching the disease when alive - if 'undercooked' the salmonella bacterium are not killed.

Salmonella can also be contracted and spread by eating contaminated food, food prepared in unhygienic conditions and food that has been stored too long at too high a temperature - allowing salmonella bacteria to multiply.

In the UK, to control the spread of the disease, poultry like chickens and turkeys are given a vaccination against salmonella.

In the US chickens are washed with a chlorine solution to kill the salmonella bacterium - a controversial and banned practice in Europe - including the UK - but post-Brexit, who knows!.

Whatever the poultry source, make sure it is well cooked, and wash your hands before and after food preparation.

Gonorrhoea is a bacterium that is classed as a sexually transmitted disease (STD).

(sexually transmitted infection, STI)

Sexually transmitted diseases are passed on by sexual contact e.g. having unprotected sex.

A person infected with gonorrhoea bacteria will experience pain on urination and other symptoms include a thick yellow or green discharge from the penis or the vagina.

Gonorrhoea was, and still is, treated with the antibiotic penicillin, but unfortunately, strains of bacteria have evolved that resist this treatment, making it less effective

The best strategy to reduce the spread of gonorrhoea is to treat patients with new antibiotics that the bacteria are not as resistant too AND to use barrier methods of contraception like condoms to prevent infection and spreading by contact.

Cholera is from a bacterium called Vibrio cholerae.

Cholera bacteria cause diarrhoea and in severe cases causes dangerous fluid loss - severe dehydration, which can cause complications which can be fatal. The main symptom of diarrhoea persists for a few days.

The principal source, and cause of spreading, is contaminated water supply containing the cholera bacterium, which is the means by which the cholera bacteria are spread.

Poor hygiene further contributes to the spread of the infection because faeces contain the cholera bacteria too.

Cholera is most prevalent in developing countries where clean water is in limited supply - if at all.

Therefore, prevention is best obtained by using a clean water supply - there were outbreaks of cholera in Victorian England until it was realised the source of the infection was dirty water!

Tuberculosis (TB) is from a bacterium called Mycobacterium tuberculosis

The effects and symptoms of the tuberculosis bacteria infection include coughing and lung damage - older people can be severely weakened and further medical complications may arise - one disease can be compounded with another.

The bacteria are conveyed through the air when infected people cough or sneeze out droplets containing the bacterium - note that coughing and sneezing are the principal symptoms!

Ways to minimise the spread of tuberculosis infection

Infected people should minimise contact with people - avoid crowded places, sleep alone.

Although we all should, it is particularly important that a TB infected person should practice good hygiene - like coughing or blowing nose into tissues and carefully disposing them, washing hands regularly, wearing a face mask if near people, and keeping the house well ventilated also reduces the chances of transmission.

Stomach ulcers can be caused by the bacterium Helicobacter pylori

Stomach ulcer symptoms and effects include stomach pain, nausea and vomiting.

The bacteria reduce the stomach's defences against the acid it produces to digest food and causes inflammation of the stomach lining and damaging it.

The stomach acid (quite concentrated hydrochloric acid) penetrates the stomach lining creating a hole (the ulcer) that exposes the stomach tissue

The bacteria are ingested after eating contaminated food or drinking contaminated water - referred to as an example of oral transmission.

The chances of getting a stomach ulcers by transmission are greatly reduced by having access to clean water and watching your personal hygiene e.g. washing hands when going to the toilet, disinfecting food preparation spaces and a clean house - especially the bathroom and kitchen.

Treatments for stomach ulcers include the use of antibiotics and drugs that reduce stomach acid production.

Chlamydia is a kind bacterium that is classed as a sexually transmitted disease (STD)

(Chlamydia is also described as sexually transmitted infection, STI)

(An STI can be caused by a bacteria, fungus, protist or virus)

Chlamydia bacteria behave like a virus, because they can only reproduce inside a living host cell.

Sexually transmitted diseases are passed on by sexual contact e.g. having unprotected sexual intercourse.

Chlamydia doesn't always produce symptoms BUT it can cause infertility in men and women.

It is possible for chlamydia to be passed on from an infected mother to her child during childbirth.

The spread of chlamydia can be reduced by ...

(i) wearing a condom during sexual intercourse,

(ii) screening individuals, so that if diagnosed, they can be treated with oral antibiotics, and this reduces the time frame in which the infected person could unknowingly pass on the infection.

(iii) refraining from sexual contact.

Escherichia coli (E. coli) bacterium

E.coli bacteria normally live in the intestines of healthy people and animals and are not usually a problem.

However, while most varieties of E. coli are harmless or cause relatively brief diarrhea, there are a few particularly nasty strains which can cause severe abdominal cramps, bloody diarrhea and vomiting.

E.coli cells can divide every 20 minutes, so their numbers can rapidly increase!

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(e) Examples of viral pathogen communicable diseases

The disease, pathogen, symptoms-effects, means of spread and how to reduce or prevent transmission are all described in the following examples.

Measles is a viral disease spread by droplets when an infected person sneezes or coughs.

The primary symptoms are a red skin rash and a higher than normal body temperature from a fever.

Measles can be a serious medical condition and even fatal if complications develop e.g. a lung infection like pneumonia or the brain infection encephalitis.

Most people are adequately protected if you are vaccinated when young.

Human Immunodeficiency Virus (HIV) a sexually transmitted disease (STD)

HIV is a virus spread by sexual contact or by exchanging bodily fluids like blood e.g. when two people share the same needle in drug taking, sexual intercourse - semen and vaginal fluids.

(HIV is described as sexually transmitted infection, STI)

(An STI can be caused by a bacteria, fungus, protist or virus)

The HIV attacks white blood cells, an important part of the body's defence system.

Usually, initially, the HIV infected person experiences flue-like symptoms for a few weeks but then no other symptoms may be experienced for several years. Some people never exhibit flue-like symptoms.

The virus enters the lymph nodes and attacks some of the white blood cells of the immune system.

Antiretroviral drugs are prescribed to inhibit the virus from entering the lymph node tissues which are rich in lymphocytes - essential component of the body's immune system.

If treatment with antiretroviral drugs is not successful, the virus enters the lymph nodes and attacks the body's immune cells.

If diagnosed in time, HIV can be controlled with antiretroviral drugs that stop the HIV virus replicating in the body.

The HIV virus attacks some of the types of white blood cells, kills them and so damaging part of the bodies immune system.

This means the body's defences against other infections, the immune system, is severely weakened and may not be able to cope, including an increased risk of cancer.

AIDS sufferers are also much more susceptible to other communicable diseases like flue.

At this advanced stage, when your body is struggling to cope with any infection at all, HIV leads to 'late stage HIV infection' known as Acquired Immune Deficiency Syndrome (AIDS).

Since HIV prevents the immune system from working properly, the body is extremely vulnerable to infection from any other pathogen - an unfortunate 'knock on' effect because the person's immune system further deteriorates and eventually fails to fight off an infection - an extremely dangerous situation.

e.g. the bacteria that cause the communicable disease tuberculosis would normally be destroyed by the body's immune system before symptoms develop. However, the immune system of someone infected with the HIV virus are much more likely to display symptoms of tuberculosis and the disease may develop very rapidly - recovery is much more difficult and the outcome can be serious e.g. persistent coughing and lung damage and possibly death.

Prevention of infection and minimising the spread of HIV and hence AIDS

HIV is spread by exchanging infected bodily fluids in sexual intercourse or sharing needles when taking drugs e.g. HIV is transported in blood, semen and vaginal fluids.

So prevention measures to minimise the spread of HIV disease include ...

(i) Using a condom during sexual intercourse

(ii) Drug users NOT sharing needles,

(iii) There are some medications (antiretroviral drugs) available to reduce the risk of passing on the infection during sex - this also applies to mothers passing on the infection to babies during pregnancy.

(iv) Being screened and following up with suitable treatment e.g. antiretroviral drugs which stop the virus reproducing.

Tobacco mosaic virus (TMV) is a virus that attacks plants.

See Plant Diseases notes for more details.

Ebola is caused by a virus which causes a fever accompanied by bleeding (haemorrhagic fever).

Ebola haemorrhagic fever (EHF) is a deadly virus - 50-90% 0f infected people die.

Ebola is carried by fruit bats (the vector) and infect other animals and humans - unfortunately, in some parts of West Africa, fruit bat is considered a delicacy - a cultural barrier in reducing transmission.

Ebola spreads through human-to-human transmission via direct contact through broken skin or mucous membranes with the blood, secretions like saliva, organs or other bodily fluids of infected people, and with surfaces and materials (e.g. bedding, clothing) contaminated with these fluids.

Ebola symptoms include fever, headache, diarrhoea, nausea and rashes - typical observations of the effect on many pathogen infections.

Ebola causes haemorrhagic fever - where you bleed internally or from your eyes, mouth or nose.

To reduce the spread of Ebola, infected people should be isolated and sterilising any areas where the virus might be, including hospital beds used by Ebola sufferers.

The safe burial of the victims of Ebola.

The spread and transmission of Ebola can be further reduced by ensuring ALL medical staff wash their hands frequently and wear protective clothing.

Other precautions include minimising risk of contact with infected animals.

Restricting travel between locations with Ebola outbreaks.

Human papillomavirus (HPV) is a virus that can infect the human reproductive system.

The HPV virus is transmitted in bodily fluids, usually during sexual activity.

Infection by the HPV virus doesn't always show symptoms and usually clears up on its own in a couple of months.

Unfortunately, sometimes the HPV infection promotes cell DNA changes causing the formation of certain types of cancer.

It is thought that most cases of cervical cancer arise from HPV infections - one disease causes another.

In this case of a communicable disease (HPV) causes the formation of a non-communicable disease (cancer).

Chicken pox is caused by a virus - typical symptom is a spotty rash.

When a person has become infected with this pathogen, it takes nearly 14 days for the rash to appear, this time is called the incubation period.

Hepatitis

The hepatitis virus causes long-term infections in the liver where it lives in the cells.

This gives you an increased chance of developing liver cancer - a case of a communicable disease making you more susceptible to a non-communicable disease.

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(f) Examples of protist pathogen communicable diseases

The disease, pathogen, symptoms-effects, means of spread and how to reduce or prevent transmission are all described in the following examples.

Malaria is caused by a protist (a type of eukaryote cell).

Malaria causes damage to red blood cells and in severe cases, the liver.

Malaria causes repeated episodes of fever which can overwhelm the bodies defences and prove fatal.

The symptoms of malaria include fever (raised temperature above normal), sweats and chills, headaches, muscle pain, diarrhoea and a cough.

Part of the malarial protist's life cycle is inside an animal - including us!

The mosquito is an animal vector - a 'carrier' - a means of spreading a disease - it conveys the protist without any ill-effect to itself - it just passes the protist on without developing the disease!

The female mosquito feeds by sucking blood from warm-blooded animals - if the animal is infected with malaria, the protist is spread when the same mosquito feeds on another animal.

When the mosquito does feed on another animal, it infects it by inadvertently passing the malarial protist into the blood stream when feeding - the mosquito's mouthparts are adapted to break through skin and suck out blood, but the act of feeding inserts the malaria protist at the same time.

Malaria is still a common infection in many parts of the world but you can do some things to combat it.

The mosquito's lifecycle is complicated.

Mosquitoes breed on standing water where the eggs hatch and the larvae develop into pupas in the water.

You can reduce the spread of malaria by stopping the mosquitoes from breeding e.g. affecting their breeding grounds by draining swamps in various ways e.g.

You can spray swamps with insecticide (biodegradable I hope).

You can spray oil on the water to stop the larvae breathing.

As a precaution, people living in areas with malaria risk should make sure any containers of standing water from rain are got rid of to minimise risk of mosquitoes breeding on the surface of the water.

For your own personal protection from insect bites and reduce the chance of spreading malarial disease you can ...

(i) use insecticide sprays to kill the mosquitoes,

(ii) treat any exposed skin with insect repellent,

(iii) employ mosquito nets over your bed at night to stop getting bitten while sleeping,

 - all of which are designed to stop mosquitoes biting you and passing the malaria disease on!

If infected, you can take an antimalarial drugs to kill the specific harmful protist in your bloodstream.

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(g) Examples of fungal pathogen communicable diseases

The disease, pathogen, symptoms-effects, means of spread and how to reduce or prevent transmission are all described in the following examples.

Rose black spot is fungus that affects rose plants. See Plant Diseases notes for more details.

Athlete's foot is a fungus which makes skin itch and flake off.

It is most commonly spreading by a person coming into contact with a surface touched by an infected person e.g. shower floors or towels.

Chalara ash dieback is a fungus that infects ash trees. See Plant Diseases notes for more details.

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(h) More on how can we prevent, or reduce the spread of communicable diseases

Early detection and treatment

This reduces chances of spreading a disease, but it is very dependant on your own knowledge of a disease and access to a good healthcare system for diagnosis and treatment.

Hygiene

You should be as hygienic as possible in your everyday life to reduce the spread of communicable diseases e.g.

washing your hands thoroughly after going to the toilet, washing your hands before preparing and handling food, sneezing into a tissue rather than into the 'open air' and disposing of it.

Treating and cleaning kitchen worktops with antiseptic sprays.

Isolation

If you are infected with a disease, as far is practicable, try to minimise contact with other people and therefore minimise passing the infection on - in extreme cases, you might end up in an isolation ward.

Vaccination

Where possible, vaccination of people or animals can be used to control the spread of communicable diseases.

Vaccination prevents the disease from developing and so the infection cannot be passed on.

See Keeping healthy - defence against pathogens, infectious diseases, vaccination for more details

Vectors

If you can kill or reduce the organisms that carry a disease (the vectors) you minimise their ability to pass the disease on.

Vectors like insects can be killed using insecticides or destroying their breeding ground habitats.

Isolating yourself, as far as is practicable, from other people, reducing your chances of passing on your infection. Some people with a serious communicable disease may be placed in a hospital isolation ward.

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(i) VIRUSES - structure, reproduction and killing cells

What are viruses?

The basic structure of a virus

Viruses are NOT cells, but usually consist of a strand of genetic material enclosed in a protein coat.

The genetic material, nucleic acid can be either DNA or RNA.

Sometimes the virus has an outer coat or envelope.

Viruses come in all shapes and sizes e.g. spherical (on the left) or various other geometrical shapes with a tail and trailing fibres (on the right).

All viruses have spikes of protein that can attach to receptors of host cells - prior to invasion!

See Introduction to plant and animal cell structure, with section on virus structure

How do they reproduce?

They cannot reproduce on their and have to 'invade' i.e. infect a living cell that acts as a host.

Specific types of viruses only infect specific cells and persuades them to reproduce the invading virus.

The sequence of events is:

The virus particle attaches to a host cell when its protein spikes recognise the receptors.

The virus releases its genetic instructions into the host cell.

The injected genetic material takes over the host cell's enzymes.

These same enzymes make parts for more new virus particles.

The new particles assemble forming new viruses.

Specific types of virus will only infect specific cells, known as host cells.

The life cycle pathways of a virus

The life cycle of a virus begins when it gets through a cell membrane to infect a host cell.

Most viruses reproduce by the lytic pathway and but others involve the lysogenic pathway first.

The lytic cycle pathway

In the lytic pathway the virus attaches itself to a specific host cell and injects its genetic material (viral DNA) into the cytoplasm through the host cell's membrane.

The host cell now contains the viral DNA.

The virus then uses the proteins and enzymes of the host cell to replicate its genetic material (DNA instructions) and so produce the material for the new viruses.

The virus components then assemble to form lots of new viruses.

Eventually so many viruses are produced that the cell bursts open, killing it - thus releasing lots more viruses to invade more host cells to create more viruses!

Therefore back to square and the cycles begin all over again, not good for the host cells!

The lysogenic cycle pathway

As with the lytic cycle, the virus attaches itself to a specific host cell and injects its genetic material (viral DNA) into the cytoplasm through the host cell's membrane.

and The host cell now contains the viral DNA, but unlike in the lytic cycle, the viral DNA becomes incorporated into the host cell's genome..

, and Therefore the viral genetic material gets replicated along with the host DNA every time the host cell divides, but for the time being, the virus stays dormant - doesn't do anything - so no new viruses are formed.

BUT, unfortunately, all the new cells produced in the lysogenic pathway are now carrying the viral genetic information and therefore, at a later time, when triggered, produce lots of viruses.

Eventually, some kind of trigger e.g. from a chemical stimulation, causes the viral genetic material to leave the genome and become separated in the host cell.

The virus then reproduces using the lytic pathway to described above.

Note: The lytic cycle can happen in a few minutes. but the viral DNA can stay in the lysogenic cycle for many years.

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(j) Evaluating data, statistics, graphs and correlation

For the moment see last section on Keeping healthy - non-communicable diseases - risk factors

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(k) Learning objectives for the pages on diseases and the body's defences

• Know that infectious diseases are caused by pathogens.
  • An infectious disease is one that spreads from one person to another.

  • Microorganisms that cause infectious disease are called pathogens.

  • Bacteria and viruses may reproduce rapidly inside the body and produce poisons (toxins) that make us feel ill.

    • Bacteria and certain protozoa are very small cells which can rapidly reproduce by cell division in your body making you feel ill by damaging your body's cells and producing toxins (poisons produced as a by-product of the bacteria's cell chemistry).

    • Viruses are NOT cells and much smaller than bacteria and damage the cells in which they reproduce.

      • Viruses replicate by invading a cell and using the cell's genetic machinery to reproduce themselves i.e. copies of the original virus.

      • The virus 'invaded' cell then bursts releasing lots of new viruses.

      • Fungi are also pathogens and includes microorganisms like yeasts and moulds (so don't eat mouldy food!).

    • (Knowledge of the structure of bacteria and viruses is not required here.)

    • Fungi are also pathogens and includes microorganisms like yeasts and moulds.

• Be able to describe how pathogens are spread, including:
  • a) in water, including cholera bacterium
    • You can be infected with a pathogen by coming into contact with contaminated water - which is why swimming bath waters are treated to kill bacteria with chlorine or ozone. In poor third world countries the bacterial infection cholera, which causes diarrhoea and dehydration, is readily spread in water contaminated with the faeces of cholera sufferers. It is potentially very serious, particularly for the very young and the very old and undernourished adults and children in poor third world countries with poor sanitation.
  • b) by food, including Salmonella bacterium infection
    • If you eat food contaminated with pathogens the resulting food poisoning effects can be very unpleasant and potentially very serious, particularly for the very young and the very old and the poor of the third world. If food is kept too long at the wrong temperature, left out in the open, or food like meat undercooked, you may be poisoned by the bacterium salmonella.
  • c) airborne (eg coughing, sneezing), including influenza virus (causes flue)
    • If you are suffering from a cough, chest infection or flue etc. and you don't take precautions with a large handkerchief or tissue, when you cough or sneeze you blast out into the air a fine mist of water droplets containing millions of bacteria or viruses. People around you breathe in you exhaled pathogens and potentially become infected. Lots of people in a crowded room are great breeding places for pathogens!
  • d) by contact, including athlete’s foot fungus infection
    • You can be infected with a pathogen just by touching a contaminated surface with e.g. your hand or foot. A common example is the spread of athlete's foot, a fungal infection easily spread in swimming bath surfaces, shower floors, towels i.e. anything an athlete's foot carrier has been in contact with.
  • e) by body fluids, including HIV infection
    • The HIV virus causes AIDS, a disease that stops our immune system from functioning properly - you become more susceptible to infectious diseases than a normal healthy person and the condition is often fatal in the end, despite the best efforts of anti-viral drugs. These kinds of pathogens can only be passed on by direct contact with body fluids from another person e.g. from a HIV carrier's sperm during sexual intercourse, or some body penetrating situation e.g. using the same drug needle as a HIV carrier.
  • f) by animal vectors (animals that spread diseases), including:
    • (i) housefly: dysentery bacterium
      • The common housefly is a carrier of a nasty protozoan bacterium. This pathogen causes dysentery, a disease that expresses itself with severe diarrhoea and dehydration. Again this can have serious consequences for the very young,  the very old and the poor of the third world.
    • (ii) Anopheles mosquito: malarial protozoan
      • The mosquito is a carrier of protozoan pathogen that causes the disease called malaria, a disease that causes potentially fatal kidney and brain damage. This serious infectious disease is passed onto another animal which is bitten by a mosquito - a mosquito bite is a bit more serious than a bee or wasp sting!
• Be able to explain how the human body can be effective against attack from pathogens, including:
  • The body has different physical and chemical ways of protecting itself against pathogens.
  • a) Physical barriers – skin, cilia, mucus

    • Physical protection from pathogens

    • Your skin and hairs and mucous in the respiratory tract can stop a lot of the pathogen cells from entering your body. The whole of the respiratory tract from the nasal passage, down the trachea and into the lungs is covered with mucous and lined cilia (fine hairs that can move freely at their ends). The mucous traps dust and bacteria before they can get down into the lungs and the cilia move the mucous along from the lungs up to the nasal passage -and then you can blow your nose!

    • Skin in good condition acts as a very effective barrier against pathogens. When a cut in the skin occurs, small sections of cells called platelets help the blood to clot quickly to seal the wound (seal = scab when dry) and prevent microorganisms entering the skin tissue or blood stream. The greater the concentration of platelets in the blood the faster the clotting process ('sealing') can occur.

  • b) Chemical defence – hydrochloric acid in the stomach, lysozymes in tears

    • Chemical protection by killing pathogens

    • In tears our eyes produce chemicals called lysozymes that kill bacterial microorganisms on the surface of the eye.

    • Your stomach contains quite concentrated hydrochloric acid which kills the majority of pathogenic bacteria - sadly not all of them at times!

• Be able to demonstrate an understanding that plants produce chemicals that have antibacterial effects in order to defend themselves, some of which are used by humans.
  • Plants attacked by pathogens can defend themselves by producing chemicals, often in oil secretions, that have antibacterial properties.
  • Some of these oils have medicinal properties that humans have used in traditional medicine recipes.
  • Other oils have been used as additives in products of the cosmetics industry.
• Be able to describe how antiseptics can be used to prevent the spread of infection.
  • Antiseptic chemicals are designed to prevent infection rather than treat and cure an existing infection - prevention is always better than a cure!
  • Antiseptics are chemicals that are applied to the outside of your body to kill pathogens like bacteria or prevent their growth.
  • Antiseptics help to prevent infection of cleaned skin wounds and the surface of the skin e.g. a larger area where a surgical operation might be done and they are also applied to surfaces where hygiene is important e.g. in the bathroom.
  • Antiseptics range from those used in the home e.g. for cuts and bruises, toilet cleaners, treating food preparation surfaces, and in GP surgeries, and in hospitals to prevent infection during operations and on hospital wards to prevent the spread of dangerous pathogens like MRSA - you should always clean your hands with the antiseptic facilities provided when visiting friends or relatives in hospital.
• Be able to explain the use of antibiotics to control infection, including:
  • Antibiotics are taken internally e.g. intravenous syringe injection, or orally taken tablet or liquid suspension.
    • In other words they are treating you from the inside and treat an existing pathogen infection you have (bacterial or fungal microorganism)
      • Compare these two point with the external use of antiseptics in preventing infection.
  • a) Antibacterials to treat bacterial infections
    • Probably the most well known antibacterial is the antibiotic penicillin which is effective against many bacterial infections BUT NOT viruses like the common cold or flue.
    • An antibiotic can kill bacteria or prevent them growing and reproducing.
  • b) antifungal to treat fungal infections
    • Antifungal chemicals kill or prevent the growth of fungi microorganisms e.g creams for the treatment of the fungal infection athlete's foot.
• Be able to evaluate evidence that resistant strains of bacteria, including MRSA, can arise from the misuse of antibiotics.

  • Antibiotics, including penicillin, are medicines that help to cure bacterial disease by killing infectious bacteria inside the body.

    • What is an antibiotic?

    • Antibiotics cannot be used to kill viral pathogens, which live and reproduce inside cells.

      • Antibiotics do not destroy viruses, typified by the cold and flue viruses we all suffer from. Viruses make your own body cells reproduce the invasive virus and unfortunately anti-viral drugs may attack good cells too!

    • Antibiotics like penicillin kill or prevent the growth of harmful pathogens, they kill the bacteria but not your own body cells.

    • Different antibiotics attack different bacteria, so it is important that specific bacteria should be treated by specific antibiotics.

    • The use of antibiotics has greatly reduced deaths from infectious bacterial diseases.

    • However, overuse and inappropriate use of antibiotics has increased the rate of development of antibiotic resistant strains of bacteria.

    • You need to be aware that it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

  • Many strains of bacteria, including MRSA, have developed resistance to antibiotics due to mutations, which cause stronger more resilient strains of bacteria to survive as a result of natural selection.

    • To prevent further resistance arising it is important to avoid over-use of antibiotics.

    • Knowledge of the development of resistance in bacteria is limited to the fact that pathogens mutate, producing resistant strains.

  • Mutations of pathogens produce new strains.

    • Antibiotics and vaccinations may no longer be effective against a new resistant strain of the pathogen.

    • The new strain will then spread rapidly because people are not immune to it and there is no effective treatment.

    • Can bacteria become resistant to antibiotics?

      • Unfortunately the answer is yes! Bacteria will sometimes quite naturally mutate into forms that are resistant to current antibiotics, so if your infected with a new strain of bacteria, your resistance is not as effective.

      • If an infection is treated with an antibiotic, any resistant bacteria will survive and this means resistant bacteria can survive and reproduce to infect other people, while the non-resistant strains will tend to be reduced.

      • This is an example of natural selection at the individual cell level and drug companies are constantly trying to develop new antibiotics to combat the new evolving strains of harmful bacteria - but new harmful 'superbugs' are becoming more common the more we use antibiotics and new epidemics can break out!

      • MRSA, methicillin-resistant staphylococcus aureus, can't be treated with many current antibiotics and causes serious wound infections that can be fatal to young babies or elderly people in particular.

      • Misuse by over-prescribing antibiotics is believed to be causing the rise of mutant resistant strains of bacteria, so doctors are being advised to avoid over-prescribing antibiotics to reduce the mutation rate and not treating mild infections with antibiotics.

      • It isn't just bacteria that can mutate, viruses can also evolve via new mutations. Viruses are notable for the rapidity with which they can mutate which makes it difficult to develop new vaccines. The reason being that changes in the virus (or bacteria) DNA leads to different gene expression in the form of different antigens, so different antibodies are needed. The flue virus is a never ending problem and in the past pandemics (epidemics across many countries at the same time) have killed millions of people, mercifully this rarely happens these days thanks to antibiotics.

      • Individual resistant pathogens survive and reproduce, so the population of the resistant strain increases.

      • Now, antibiotics are not used to treat non-serious infections, such as mild throat infections, so that the rate of development of resistant strains is slowed down.

• Revise any investigation into the effects of antiseptics or antibiotics on microbial cultures.

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General HUMAN BIOLOGY revision notes

See also cell biology index above

Introduction to the organisation of cells => tissues => organs => organ systems (e.g. in humans)

Examples of surfaces for the exchange of substances in animal organisms   gcse biology revision notes

See also Enzymes - section on digestion and synthesis  gcse biology revision notes

The human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

Homeostasis - introduction to how it functions (negative feedback systems explained)  gcse biology revision notes

Homeostasis - control of blood sugar level - insulin and diabetes  gcse biology revision notes

Homeostasis - osmoregulation, ADH, water control, urea and ion concentrations and kidney function, dialysis

Homeostasis - thermoregulation, control of temperature  gcse biology revision notes

The brain - what the different parts do and the dangers if damaged gcse biology revision notes

An introduction to the nervous system including the reflex arc  gcse biology revision notes

Hormone systems - Introduction to the endocrine system - adrenaline & thyroxine hormones  gcse biology revision

Hormone systems - menstrual cycle, contraception, fertility treatments  gcse biology revision notes

Respiration - aerobic and anaerobic in plants and animals.  gcse biology revision notes

Keeping healthy - communicable diseases - pathogen infections   gcse biology revision notes

Keeping healthy - non-communicable diseases - risk factors for e.g. cancers   gcse biology revision notes

Keeping healthy - diet and exercise  gcse biology revision notes

Keeping healthy - defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies

See also Culturing microorganisms like bacteria - testing antibiotics/antiseptics  gcse biology revision

Food tests for reducing sugars, starch, proteins and lipids  gcse biology revision notes

The eye - structure and function - correction of vision defects  gcse biology revision notes

Optics - lens types (convex, concave, uses), experiments, ray diagrams, correction of eye defects (gcse physics)