Communicable (infectious) diseasesSpec 4.3.1.1
In short
A pathogen is a microorganism that causes infectious disease, and may be a virus, bacterium, protist or fungus. Communicable diseases are spread by direct contact, by water or by air. Bacteria and viruses may reproduce rapidly inside the body. Bacteria may produce toxins that damage tissues, and viruses live and reproduce inside cells, causing cell damage.
Pathogens are microorganisms that cause infectious disease. Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals.
Pathogens depend on their host to provide the conditions and nutrients they need to grow and reproduce.
How pathogens spread
Pathogens can be spread by direct contact, by water or by air.
| Route | Examples |
|---|---|
| Direct contact | HIV (sexual contact, exchange of body fluids such as blood); gonorrhoea (sexual contact) |
| Water | Rose black spot in plants (spread in the environment by water) |
| Air | Measles (inhaling droplets from sneezes and coughs); rose black spot (spread by wind) |
| Other routes in the spec | Salmonella (in food, or on food prepared in unhygienic conditions); malaria (carried by mosquitos) |
How pathogens make us ill
- Bacteria and viruses may reproduce rapidly inside the body.
- Bacteria may produce poisons called toxins that damage tissues and make us feel ill.
- Viruses live and reproduce inside cells, causing cell damage.
Reducing or preventing the spread
To explain how spread can be reduced, link the method to the way the disease is passed on.
- Good hygiene, such as washing hands and preparing food hygienically, reduces spread by contact and in food.
- Covering the mouth and nose when coughing or sneezing, and keeping infected people away from others, reduces spread through the air.
- Vaccination prevents people from becoming infected.
- Controlling vectors, such as stopping mosquitos from breeding, and avoiding bites reduces the spread of malaria.
- Barrier contraception (a condom) and not sharing needles reduce the spread of sexually transmitted and blood-borne diseases.
- Treating infected people, for example with antibiotics for gonorrhoea, reduces the chance of them passing the pathogen on.
- In plants, destroying infected leaves and using fungicides reduce the spread of fungal disease.
For a 'how can the spread be reduced' question, name the method and say why it works. 'Wash hands' alone is not enough; add 'to remove pathogens that could be passed on by contact'.
- Pathogen
- A microorganism that causes infectious disease.
- Toxin
- A poison produced by bacteria that damages tissues and makes us feel ill.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026