Communicable diseases

Pathogens are microorganisms that cause infectious disease. They may be viruses, bacteria, protists or fungi, they may infect plants or animals, and they can be spread by direct contact, by water or by air.

Bacteria and viruses may reproduce rapidly inside the body, and the specification gives a different mechanism for each. Bacteria may produce poisons — toxins — that damage tissues. Viruses live and reproduce inside cells, causing cell damage.

Note the word "may". This is a useful way to keep the two apart, and it is what the exam expects you to write, but it is not a law: not every bacterium produces toxins, and some of the damage a virus does comes from your own immune response to it rather than from the virus directly. Give the mechanism the specification gives, and do not stretch it into a claim about every pathogen.

You are also expected to explain how the spread of disease can be reduced or prevented, and the answers come from the routes above. Break the route and you break the spread:

• Hygiene — washing hands, preparing food safely, disinfecting surfaces. • Isolating infected individuals, so there is no direct contact. • Destroying vectors, the organisms that carry a pathogen from one host to another. • Vaccination, which stops the pathogen establishing itself in the first place.

A question naming a specific disease wants the measure that fits how that disease travels — a condom for a sexually transmitted infection, a mosquito net for malaria.

Three viral diseases are named, and you need symptoms, spread and control for each.

Measles shows symptoms of fever and a red skin rash. It is a serious illness that can be fatal if complications arise, which is why most young children are vaccinated against it. The measles virus is spread by inhalation of droplets from sneezes and coughs.

HIV initially causes a flu-like illness. Unless it is successfully controlled with antiretroviral drugs, the virus attacks the body's immune cells. Late stage HIV infection, or AIDS, occurs when the immune system becomes so badly damaged that it can no longer deal with other infections or cancers. HIV is spread by sexual contact, or by exchange of body fluids such as blood when drug users share needles.

Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species including tomatoes. It gives a distinctive 'mosaic' pattern of discolouration on the leaves, which affects the growth of the plant because there is less photosynthesis.

Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions. Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete. In the UK, poultry are vaccinated against Salmonella to control the spread.

Gonorrhoea is a sexually transmitted disease with symptoms of a thick yellow or green discharge from the vagina or penis, and pain on urinating. It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared. It is spread by sexual contact, and the spread can be controlled by treatment with antibiotics or by using a barrier method of contraception such as a condom.

Note what is being vaccinated in the Salmonella case: the poultry, not the people. Controlling a disease at its source is a legitimate answer to "how could the spread be reduced", and it is the one this example is here to teach.

Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early. It affects the growth of the plant because photosynthesis is reduced.

It is spread in the environment by water or wind. It can be treated by using fungicides, and by removing and destroying the affected leaves — destroying rather than composting, so the spores do not simply return.

The pathogens that cause malaria are protists. The malarial protist has a life cycle that includes the mosquito, which acts as the vector — it carries the pathogen from one person to another without being ill itself.

Malaria causes recurrent episodes of fever and can be fatal. The spread is controlled by preventing the vectors, mosquitos, from breeding, and by using mosquito nets to avoid being bitten.

The body's first line is non-specific — it works against anything, rather than against one pathogen. Four defences are named, and each has a mechanism worth stating rather than just naming.

If a pathogen gets past all of that and enters the body, the immune system tries to destroy it. White blood cells defend against pathogens in three ways: phagocytosis, antibody production and antitoxin production.

In phagocytosis the white blood cell engulfs the pathogen and digests it. Antibodies are proteins that lock onto the antigens on a particular pathogen's surface, so they are specific — an antibody for measles does nothing against gonorrhoea. Antitoxins neutralise the toxins that bacteria produce.

Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body, to stimulate the white blood cells to produce antibodies. If the same pathogen re-enters the body, the white blood cells respond quickly to produce the correct antibodies, preventing infection.

Two things to explain, and questions usually want both: how a vaccine protects the individual, and how immunising a large proportion of the population reduces the spread of a pathogen. The second is the one students forget — if most people cannot catch it, the pathogen runs out of hosts to move between, which protects even those who are not vaccinated.

You may also be asked to evaluate the global use of vaccination in the prevention of disease, which means weighing both sides and reaching a judgement. In favour: vaccination has eradicated smallpox worldwide and made diseases such as measles and polio rare where uptake is high; it prevents epidemics when enough of a population is immune; and it is far cheaper than treating the disease. Against, or at least complicating it: no vaccine works in everyone, some people cannot be vaccinated for medical reasons, side effects occur in a small number of cases, and cold storage and delivery make global coverage hard and expensive. Falling uptake in one community can bring a disease back for everyone.

You do not need details of vaccination schedules or the side effects of specific vaccines.

Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. Specific bacteria should be treated by specific antibiotics — the right one for the infection, not any one to hand.

The use of antibiotics has greatly reduced deaths from infectious bacterial diseases. However, the emergence of strains resistant to antibiotics is of great concern, which is why they are no longer prescribed for every complaint.

Antibiotics cannot kill viral pathogens. Painkillers and other medicines treat the symptoms of disease but do not kill pathogens — they make you feel better while your immune system does the work. And it is difficult to develop drugs that kill viruses without also damaging the body's tissues, because viruses reproduce inside your own cells: killing the virus tends to mean killing the cell around it.

Traditionally drugs were extracted from plants and microorganisms. Three examples are named: the heart drug digitalis originates from foxgloves, the painkiller aspirin originates from willow, and penicillin was discovered by Alexander Fleming from the Penicillium mould.

Most new drugs are now synthesised by chemists in the pharmaceutical industry, though the starting point may still be a chemical extracted from a plant.

New medical drugs have to be tested and trialled before being used, to check that they are safe and effective. They are extensively tested for three things: toxicity, efficacy and dose — is it poisonous, does it work, and how much should be given.

In a double blind trial neither the patient nor the doctor knows who has the real drug and who has the placebo, which stops either of them influencing the result. And the results of testing and trials are published only after scrutiny by peer review — other scientists check the work before anyone acts on it, which is what stops a false or exaggerated claim entering medicine.