Health and communicable disease

Health is the state of physical and mental well-being. Diseases, both communicable and non-communicable, are major causes of ill health, but other factors including diet, stress and life situations may have a profound effect on both physical and mental health.

Different types of disease may interact, and the four examples in the specification are worth learning as a set:

• Defects in the immune system mean an individual is more likely to suffer from infectious diseases. • Viruses living in cells can be the trigger for cancers. • Immune reactions initially caused by a pathogen can trigger allergies such as skin rashes and asthma. • Severe physical ill health can lead to depression and other mental illness.

This section is unusually mathematical for biology. You are expected to translate disease incidence information between graphical and numerical forms, construct and interpret frequency tables, bar charts and histograms, and use a scatter diagram to identify a correlation between two variables.

A scatter diagram shows correlation: as one variable rises the other tends to rise (positive), tends to fall (negative), or shows no pattern. Correlation is not proof of cause — that distinction is the whole point of the next section.

You also need the principles of sampling as applied to scientific data, including epidemiological data. A sample must be large enough that chance variation evens out, and representative of the population being studied — a survey of one hospital's patients tells you about that hospital, not about the country. Whenever you are asked why a conclusion is unreliable, sample size and sample bias are the two places to look first.

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.

A named list of infections has to be learnt, and each one has to be learnt as a set: the type of pathogen, what it does to the host, how it spreads, and how that spread is reduced. Questions almost always ask for the last two together, because the way to stop a disease follows directly from the way it travels.

Sexually transmitted infections are spread by sexual contact, and two are named. Chlamydia is caused by a bacterium; it often produces no symptoms at all, which is what makes it spread so easily, and untreated it can cause infertility. HIV is caused by a virus; it attacks the cells of the immune system, and when enough of them have been destroyed the body can no longer fight off other infections — the stage called AIDS.

Both are reduced by using a condom, by screening and treating people who may have been exposed, and by limiting the number of sexual partners. HIV also spreads through blood, so it is reduced further by screening donated blood and by not sharing needles, and medication given to an infected mother greatly lowers the chance of passing it to her baby.

A virus cannot reproduce on its own. It has no cytoplasm, no ribosomes and no way of releasing energy, so it can only make copies of itself inside a living host cell, using that cell's machinery. There are two pathways it can take, and the difference between them explains why some infections make you ill straight away and others do not.

In the lytic pathway the virus attaches to the host cell and injects its genetic material. That genetic material takes over the cell, which stops doing its own work and instead makes viral proteins and copies of the viral genetic material. The parts assemble into new virus particles, and the cell then bursts — lysis — releasing them to infect more cells. The host cell is destroyed, and it is that destruction of cells that produces the symptoms of the illness.

In the lysogenic pathway the injected genetic material is instead inserted into the host cell's own DNA, where it is called a provirus. It stays there, doing nothing. Every time the host cell divides, the provirus is copied along with the rest of the DNA, so it spreads through a growing population of cells without any of them being harmed. No new virus particles are made and there are no symptoms.

A trigger — a chemical signal, or stress on the cell — can activate the provirus. It leaves the host DNA and the lytic pathway begins, which is why an infection can be silent for years and then suddenly flare up.