Communicable Diseases
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Examples explained, transmission, treatments, reduction, prevention

What is a communicable disease? What is a vector? How do you prevent the spread of
communicable diseases? Describe and explain an example of a
communicable disease?
Sub-index for this page
(a)
Introduction to communicable diseases
(b)
Which types
of pathogens causes communicable
diseases?
(c)
How are pathogens spread?
(d)
Examples of
bacterial pathogen communicable diseases
(e)
Examples of viral pathogen communicable diseases
(f)
Examples of
protist pathogen communicable diseases
(g)
Examples of
fungal pathogen communicable diseases
(h)
More on how can we prevent, or reduce
the spread of communicable diseases?
(i)
Viruses - structure, reproduction and cell destruction
(j)
Evaluating data,
statistics, graphs and correlation
(k)
Learning objectives for the pages on diseases and the body's defences
See also
non-communicable diseases and
plant
diseases
and
Keeping healthy - our defences against
pathogens, fighting infectious diseases, vaccination, monoclonal antibodies
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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.
TOP OF PAGE
and sub-index
(j)
Evaluating data,
statistics, graphs and correlation
For the moment see last section on Keeping healthy -
non-communicable diseases - risk factors
TOP OF PAGE
and sub-index
(k) Learning objectives for the pages on diseases and the body's defences
- Know that infectious diseases are caused by pathogens.
-
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 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.
TOP OF PAGE
and sub-index
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)
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