GCSE level School biology notes: Communicable diseases and risk factors and prevention

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Communicable Diseases - one person can infect another

Examples explained, transmission, treatments, reduction, prevention

Doc Brown's biology exam revision study notes

 What is a communicable disease?   What is a vector?

How can we minimise the spread of communicable diseases?

How do you prevent the spread of communicable diseases?

Describe and explain examples of communicable diseases?

Sub-index for this page

(1) Introduction to communicable diseases

(2) Which types of pathogens cause communicable diseases?

(3) How are pathogens spread?

(4) Bacterial pathogen communicable diseases - salmonella, E. coli

(5) Bacterial pathogen communicable diseases - cholera, tuberculosis (TB), stomach ulcers

(6) Bacterial pathogen communicable diseases - gonorrhoea, chlamydia (STDs)

(7) Viral pathogen communicable diseases - measles, chicken pox

(8) Viral pathogen communicable diseases - HIV (aids), HPV (both sexually transmitted diseases)

(9) Viral pathogen communicable diseases - ebola, hepatitis

(10) Pathogens that attack plants

(11) Examples of protist pathogen communicable disease - malaria

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

See also

Viruses - structure, reproduction and cell destruction

non-communicable diseases

Keeping healthy - our defences against pathogens, fighting infectious diseases, vaccination

For evaluating data, statistics, graphs and correlation see last section on Keeping healthy - non-communicable diseases - risk factors

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 and only use when necessary and complete the course of treatment.

      • 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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