1. Bacteria: Single-celled prokaryotes that can cause a wide range of diseases, from food poisoning to pneumonia.

2. Viruses: Non-living infectious agents that require a host cell to replicate. They can cause diseases like the flu, HIV, and COVID-19.

3. Fungi: Eukaryotic organisms that can cause infections like athlete's foot, ringworm, and yeast infections.

4. Parasites: Organisms that live in or on a host and benefit at the host's expense. This group includes protozoa, helminths (worms), and ectoparasites like ticks and lice.

5. Prions: Infectious proteins that can cause fatal neurodegenerative diseases like mad cow disease.

6. Viroids: Small, circular RNA molecules that infect plants and cause a variety of diseases.

It's worth noting that some sources might categorize these groups slightly differently. For example, some might combine protozoa and helminths into a single category of "parasitic worms" or "helminths."