A pathogen is an organism that causes disease. There are four types of pathogen: viruses, bacteria, fungi and protists.
Viruses are much smaller than cells. They can only reproduce inside the living cells of a host, and HIV is an example. Bacteria are single-celled organisms that reproduce rapidly inside the body. Cholera and tuberculosis are each caused by a bacterium.
Fungi include moulds and yeasts. Chalara ash dieback is caused by a fungus that infects ash trees. Protists are mostly single-celled organisms. Some of them are parasites, and malaria is caused by a protist.