Communicable Diseases and Pathogens

In the last lesson you learned that communicable diseases are caused by pathogens — microorganisms that cause disease — and can spread from one host to another. This lesson examines the four types of pathogen (bacteria, viruses, fungi and protists), exactly how they make us ill, the named example diseases you must know, the ways pathogens spread, and the practical steps we take to reduce or prevent that spread. This is core knowledge for the health strand of Topic B6 and underpins the next two lessons on defence and treatment.

By the end of this lesson you should be able to name the four types of pathogen, explain how bacteria and viruses damage the body, recall a named disease caused by each pathogen type, describe the main routes by which pathogens spread, and suggest practical ways to limit that spread.

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The Four Types of Pathogen

A pathogen is any microorganism that causes disease. There are four types you need to know.

Pathogen type	Key features	Example diseases
Bacteria	Small living cells (prokaryotic); reproduce very rapidly inside the body; can release toxins	Salmonella (food poisoning); tuberculosis; gonorrhoea
Viruses	Not cells; extremely small; reproduce only inside living host cells, damaging them	Measles; HIV; influenza; tobacco mosaic virus (a plant virus)
Fungi	Some single-celled, some multicellular with hyphae; can grow on or in the body	Athlete's foot; rose black spot (a plant disease)
Protists	Single-celled eukaryotes; often spread by a vector	Malaria (spread by mosquitoes)

Notice that pathogens affect plants as well as animals — tobacco mosaic virus and rose black spot are both plant diseases that OCR expects you to recognise.

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How Pathogens Make Us Ill

Different pathogens cause harm in different ways, and the contrast between bacteria and viruses is examined particularly often.

Bacteria

Bacteria are living cells that, once inside the body, reproduce very rapidly by dividing. As their numbers grow they can damage tissues directly, but the main way many bacteria make us feel ill is by producing toxins — poisonous chemicals that damage cells and tissues and cause the symptoms of the disease (such as a fever or sickness).

Viruses

Viruses are not cells and cannot reproduce on their own. Instead, a virus invades a living host cell and takes over its machinery, forcing the cell to make many copies of the virus. The host cell is then damaged or bursts to release the new viruses, which go on to infect more cells. It is this damage to the host's own cells that makes a viral disease such as measles or flu so unpleasant.

Exam Tip: The single most important contrast to learn is: bacteria reproduce rapidly and release toxins; viruses live and reproduce inside cells, damaging them. This one sentence answers a large number of OCR questions about how pathogens cause disease.

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Named Example Diseases

OCR expects you to recall specific diseases matched to their pathogen type. Learn this set.

Viral diseases

• Measles — a serious viral disease spread by droplets when an infected person coughs or sneezes. Symptoms include fever and a red skin rash; it can lead to serious complications, which is why children are vaccinated against it.
• HIV (human immunodeficiency virus) — initially causes flu-like symptoms, then a long hidden phase. HIV attacks the body's immune cells. Without treatment it can develop into AIDS, where the immune system is so damaged it can no longer fight off other infections. HIV is spread through body fluids (such as blood, or during unprotected sex) and is controlled with antiretroviral drugs.
• Tobacco mosaic virus (TMV) — a plant virus that gives a distinctive mosaic pattern of discoloration on the leaves of tobacco and other plants. The pale patches contain less chlorophyll, so the plant photosynthesises less and grows poorly.

Bacterial diseases

• Salmonella — a bacterium that causes food poisoning. It is taken in with food that has been contaminated, often by poor hygiene or undercooking. The bacteria and the toxins they produce cause fever, stomach cramps, vomiting and diarrhoea. In the UK, poultry are often vaccinated against salmonella to reduce its spread.

Fungal diseases

• Rose black spot — a fungal plant disease that produces purple or black spots on the leaves of roses. Affected leaves often turn yellow and drop early, reducing photosynthesis and weakening the plant. It spreads in water and on the wind, and is treated with fungicides or by removing affected leaves.
• Athlete's foot — a fungal infection of human skin (between the toes) causing itchy, flaky, sore skin. It spreads by contact, for example via changing-room floors, and is treated with antifungal creams.

Protist diseases

• Malaria — caused by a protist that is spread from person to person by a mosquito vector. When an infected mosquito bites a person it passes the protist into their blood; the parasite then damages red blood cells, causing recurring episodes of fever that can be fatal. Because malaria depends on the mosquito, much prevention focuses on stopping mosquitoes breeding and biting (mosquito nets, removing standing water, insecticides).

Exam Tip: Learn at least one disease for each of the four pathogen types, and be careful which are plant diseases. A classic error is to call tobacco mosaic virus or rose black spot a human disease — both affect plants.

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How Pathogens Spread

To cause disease, a pathogen must get from one host to the next. The main routes are: