Infection, and the response.

Part 1What a pathogen is

A pathogen is a microorganism that causes disease. Diseases caused by pathogens can spread from one person (or organism) to another, so they're called communicable (infectious) diseases. There are four types you must know: bacteria, viruses, fungi and protists.

Bacteria are small living cells that reproduce rapidly inside the body. They make you ill by producing toxins (poisons) that damage tissues. Viruses are much smaller and are not really cells. They reproduce by getting inside your body's cells and using them to make copies of the virus — the cell then bursts, releasing the new viruses and damaging your tissues.

Fungi can be single cells or have a body made of thread-like hyphae; these can grow and spread through tissues, and produce spores. Protists are single-celled organisms; many that cause disease are parasites spread by a vector (an organism that carries the pathogen without getting the disease itself).

Part 2How disease spreads — and how to stop it

Pathogens spread in a handful of ways. Learn the route, because each one points to a way of stopping the spread.

Direct contact — touching an infected surface, person or plant (e.g. some skin and plant diseases). Water — pathogens picked up from dirty drinking water. Air (droplets) — breathing in droplets from coughs and sneezes (e.g. measles). To reduce or prevent spread you break the route: better hygiene (washing hands, disinfecting), destroying vectors, isolating infected individuals, and vaccination.

Part 1Viral diseases

Three viral diseases to learn: measles, HIV, and a plant one, tobacco mosaic virus.

Measles spreads by droplets from coughs and sneezes. Symptoms are fever and a red skin rash. It can be very serious — even fatal — and most young children are now vaccinated against it.

HIV spreads through sexual contact or by exchanging body fluids (for example sharing needles). At first it causes a flu-like illness. Unless controlled with antiretroviral drugs, the virus attacks the body's immune cells. Once the immune system is badly damaged the condition is called AIDS, and the body can no longer fight off other infections.

Tobacco mosaic virus (TMV) is a plant disease. It gives leaves a mosaic pattern of discolouration. Because the affected areas can't photosynthesise well, the plant grows poorly.

Part 2Bacterial, fungal & protist diseases

Salmonella (bacteria) causes food poisoning — fever, stomach cramps, vomiting and diarrhoea, caused by the toxins the bacteria release. You catch it from food prepared in unhygienic conditions or from infected poultry. In the UK, poultry are vaccinated against salmonella to control it.

Gonorrhoea (bacteria) is a sexually transmitted disease (STD). Symptoms include a thick yellow/green discharge and pain when urinating. It was treated with the antibiotic penicillin until resistant strains appeared. Spread is controlled by treatment with antibiotics and by using a barrier method of contraception such as a condom.

Rose black spot (a fungus) causes purple or black spots on rose leaves; the leaves then turn yellow and drop early, so the plant photosynthesises less and grows badly. It spreads in the environment by water and wind. Gardeners treat it with fungicides and by removing and destroying affected leaves.

Malaria is caused by a protist. The protist has a life cycle that includes the mosquito, which acts as its vector — mosquitoes spread the protist to people when they feed on blood. Malaria causes recurring episodes of fever and can be fatal. Spread is reduced by stopping the mosquitoes breeding and by using mosquito nets to avoid being bitten.

Part 1Keeping pathogens out

Your first line of defence stops pathogens entering at all. These are non-specific — they work against everything.

The skin is a physical barrier; it also makes antimicrobial substances and is covered in helpful microbes. If it's cut, the blood clots to seal the wound. The nose has hairs and mucus that trap particles and pathogens in the air you breathe in. The trachea and bronchi also make mucus to trap pathogens, and tiny hairs called cilia waft that mucus up and out of the lungs. The stomach produces hydrochloric acid, which kills most pathogens you swallow in food and drink.

Part 2The immune system & white blood cells

If a pathogen gets past the barriers, your immune system deals with it. The key cells are white blood cells, and they have three jobs:

1. Phagocytosis — some white blood cells engulf and digest pathogens. 2. Producing antibodies — others make antibodies, special proteins that lock onto a specific pathogen and mark it for destruction. 3. Producing antitoxins — others make antitoxins that neutralise the toxins released by bacteria.

Antibodies are specific: a particular antibody only fits one type of pathogen, a bit like a key fitting one lock. That specificity is the basis of how vaccines work (Topic 4).

Part 1The four-step story

A vaccine contains small quantities of dead or inactive forms of a pathogen. Because the pathogen is dead or inactive, it can't make you ill — but it still carries the markers your immune system recognises. Learn this sequence; it's a classic exam answer.

1. The vaccine puts dead/inactive pathogen into the body. 2. White blood cells respond by producing the correct antibodies. 3. Some of these become memory cells that stay in the body. 4. If the live pathogen ever invades, the memory cells make the right antibodies quickly and in large amounts, destroying the pathogen before you become ill. You are now immune.

Part 2Herd immunity

If a large proportion of the population is vaccinated, the spread of a pathogen is greatly reduced — there are very few people for it to infect and pass between. This is herd immunity, and it even protects the few people who aren't vaccinated (for example, those too young or too ill), because the disease can't get a foothold.

Part 1Antibiotics vs painkillers

Antibiotics, such as penicillin, are medicines that kill bacteria inside the body (without harming your own cells). Specific bacteria need specific antibiotics, so the right one has to be used. Antibiotics have hugely reduced deaths from bacterial diseases.

But antibiotics do not kill viruses. A virus lives and reproduces inside your own cells, so it's very hard to develop a drug that destroys the virus without also damaging your cells. That's why you're not given antibiotics for a cold or flu — they're viral.

Painkillers (and other medicines that treat symptoms) only relieve symptoms such as pain — they do not kill the pathogen. The illness goes away when your immune system deals with the pathogen, not because of the painkiller.

Part 2Antibiotic resistance

Some bacteria have changed (mutated) so that an antibiotic no longer kills them — they are resistant. When you take an antibiotic, it kills the non-resistant bacteria but any resistant ones survive, reproduce, and pass on their resistance. Over time, resistant strains spread. MRSA is a well-known example.

To slow this down: doctors shouldn't prescribe antibiotics for non-serious or viral infections, and patients should complete the whole course so that all the bacteria — including any partly-resistant ones — are killed. The development of new antibiotics is slow and costly, so it isn't keeping pace with resistance.

Part 1Traditional drugs from nature

Many medicines were originally extracted from plants and microorganisms. Three examples to learn:

Penicillin was discovered by Alexander Fleming from the Penicillium mould. Today most new drugs are made by chemists in the pharmaceutical industry, but the starting point may still be a chemical found in a plant.

Part 2Testing a new drug

Before any new drug reaches patients it must be tested for three things: that it is safe (low toxicity), that it actually works (efficacy), and the correct dose. Testing happens in two stages.

Preclinical testing is done in a laboratory on cells, tissues and live animals. If the drug looks promising, it moves to clinical trials using healthy volunteers and patients. A very low dose is used at first to check it is safe, and the dose is gradually increased to find the optimum dose.

In a trial, some patients are given the new drug and others are given a placebo — a dummy treatment with no active drug — so the real effect of the drug can be compared fairly. Trials are usually double-blind: neither the patient nor the doctor knows who has the real drug until the end, which stops their expectations biasing the results. New drugs are also peer reviewed before publication to check the science.