What is a pathogen?Spec 5.4
In short
A pathogen is a disease-causing organism. The four types are viruses, bacteria, fungi and protists. Viruses are not cells and can only reproduce inside a host cell, for example HIV. Bacteria are single-celled with no nucleus, such as the cholera bacterium. A fungus causes Chalara ash dieback, and protists are single-celled with a nucleus, such as the malaria protist.
A pathogen is a disease-causing organism. The four types you need to know are viruses, bacteria, fungi and protists.
Types of pathogen| Type | Key facts | Example |
|---|
| Virus | Not a cell; much smaller than bacteria; can only reproduce inside a host cell | HIV |
|---|
| Bacterium | A single-celled organism with no nucleus | Cholera bacterium |
|---|
| Fungus | Includes single-celled and many-celled organisms | Chalara ash dieback fungus |
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| Protist | A single-celled organism with a nucleus | Malaria protist |
|---|
Exam tip:Link each pathogen type to an example from the spec. Examiners often ask you to name the type of pathogen that causes a named disease.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026
Frequently asked questions
How do pathogens spread?
Pathogens spread in several ways: in contaminated water, like cholera; through the air in droplets or spores, like tuberculosis and Chalara ash dieback; by an animal vector, like the mosquito carrying malaria; and by sexual contact, like chlamydia and HIV. To reduce the spread, identify the route and then block it.
What is the difference between bacteria and viruses?
Bacteria are single-celled organisms with no nucleus, while viruses are not cells at all. Viruses are much smaller than bacteria and can only reproduce inside a host cell. Examples are the cholera bacterium and HIV. This is why antibiotics, which inhibit cell processes in bacteria, do not work on viruses.
How can the spread of malaria be reduced?
The spread of malaria can be reduced by controlling its vector, the mosquito, which carries the malaria protist from person to person. Draining standing water and using insecticides kill mosquitoes, while mosquito nets and insect repellent stop people being bitten. Mosquitoes do not cause malaria; the protist does.
All 5 questions on Pathogens and infections
