Communicable Diseases

Pathogens

A pathogen is a microorganism that causes an infectious (communicable) disease. There are four types — viruses, bacteria, fungi and protists — and between them they infect both plants and animals.

Pathogens pass from one organism to another in three main ways: through the air (for example, droplets from coughs and sneezes), through water, and by direct contact.

Once inside the body, bacteria and viruses can multiply very quickly. They make you ill in different ways:

• Bacteria can release toxins that damage tissues.
• Viruses live and reproduce inside cells, damaging or bursting them.

The spread of disease can be reduced by good hygiene, destroying or controlling vectors, isolating infected individuals and vaccination.

Viral diseases

• Measles — causes a fever and a red skin rash. It spreads when droplets from an infected person's sneezes or coughs are breathed in. It can be fatal if complications develop, which is why most young children are vaccinated against it.
• HIV — begins with a flu-like illness. Without antiretroviral drugs, the virus attacks the body's immune cells. Once the immune system is badly damaged it can no longer fight other infections or cancers; this late stage is called AIDS. HIV spreads through sexual contact and the exchange of body fluids such as blood, for example when drug users share needles.
• Tobacco mosaic virus (TMV) — a plant pathogen that infects many species, including tomatoes. It causes a mottled, mosaic pattern of discolouration on the leaves. The discoloured areas cannot photosynthesise properly, so the plant's growth is reduced.

Bacterial diseases

• Salmonella — a cause of food poisoning, picked up by eating contaminated food or food prepared in unhygienic conditions. Fever, stomach cramps, vomiting and diarrhoea are caused by toxins the bacteria release. In the UK, poultry are vaccinated against Salmonella to limit its spread.
• Gonorrhoea — a sexually transmitted disease. Symptoms include a thick yellow or green discharge from the vagina or penis and pain when urinating. It used to be treated easily with penicillin, but many strains are now resistant. Its spread is controlled by antibiotic treatment and by using barrier contraception such as condoms.

Fungal diseases

Rose black spot produces purple or black spots on rose leaves. The leaves often turn yellow and fall early, so the plant photosynthesises less and grows poorly. The spores spread in water and wind. It is treated with fungicides and by removing and destroying infected leaves.

Protist diseases

Malaria is caused by a protist. Part of the protist's life cycle takes place inside mosquitoes, which act as vectors — they carry the pathogen between people. Malaria causes repeated bouts of fever and can be fatal.

Its spread is controlled by stopping mosquitoes breeding (for example, by draining standing water) and by using mosquito nets to prevent bites.

Human defence systems

The body's first line of defence is non-specific — it tries to stop any pathogen getting in:

• the skin forms a physical barrier and produces antimicrobial secretions,
• hairs and mucus in the nose trap particles,
• the trachea and bronchi produce mucus that traps pathogens, and cilia sweep it up to the throat to be swallowed,
• the stomach produces hydrochloric acid, which kills most swallowed pathogens.

If a pathogen gets past these, the immune system responds. White blood cells destroy pathogens in three ways:

• phagocytosis — engulfing and digesting the pathogen,
• producing antibodies, which lock on to specific antigens on the pathogen's surface,
• producing antitoxins, which neutralise the toxins released by bacteria.

Keep antibodies and antitoxins separate in your answers: antibodies target the pathogen, antitoxins target its toxins.

Vaccination

A vaccine introduces a small amount of a dead or inactive form of a pathogen into the body. This stimulates white blood cells to produce the matching antibodies. If the live pathogen enters later, the white blood cells recognise it and produce the correct antibodies quickly, so the infection is stopped before the person becomes ill.

When a large proportion of a population is vaccinated, the pathogen struggles to spread, which also protects people who have not been vaccinated. This is known as herd immunity.

Antibiotics and painkillers

Antibiotics, such as penicillin, kill bacteria inside the body. Particular bacteria need particular antibiotics, so the right one must be chosen. Antibiotics have saved huge numbers of lives, but the emergence of resistant strains of bacteria is a growing problem.

Antibiotics have no effect on viruses. Because viruses live inside the body's own cells, it is difficult to develop drugs that kill them without also damaging the body's tissues.

Painkillers and many other medicines only relieve symptoms — they do not kill the pathogen.

Discovery and development of drugs

Many drugs were first extracted from plants and microorganisms:

• digitalis, a heart drug, comes from foxgloves,
• aspirin, a painkiller, was developed from a chemical found in willow,
• penicillin was discovered by Alexander Fleming, from the Penicillium mould.

Today most new drugs are made by chemists in the pharmaceutical industry, although the starting point may still be a substance found in a plant.

Every new drug is tested for toxicity (is it safe?), efficacy (does it work?) and dose (how much is needed?):

• Preclinical testing is carried out in the laboratory on cells, tissues and live animals.
• Clinical trials then use healthy volunteers and patients. Very low doses are given first to check safety; if the drug is safe, further trials find the optimum dose.
• In a double-blind trial, some patients receive a placebo, and neither the patients nor the doctors know who received which until the trial ends. This removes bias.

Results are peer reviewed by other scientists before publication, to help prevent false or exaggerated claims.