What is a pathogen?Spec 5.4
A pathogen is a disease-causing organism. The four types you need to know are viruses, bacteria, fungi and protists.
| 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 |
| Protist | A single-celled organism with a nucleus | Malaria protist |