Pathogens can be:

* Bacteria: Single-celled organisms that can cause a wide range of infections, from pneumonia to food poisoning.

* Viruses: Tiny infectious agents that can only replicate inside the cells of living organisms. They can cause illnesses like the flu, HIV, and COVID-19.

* Fungi: Simple organisms that can cause skin infections, respiratory infections, and other illnesses.

* Parasites: Organisms that live in or on another organism and benefit at its expense. They can cause infections like malaria, hookworm, and tapeworm.

* Prions: Misfolded proteins that can cause fatal brain diseases like Creutzfeldt-Jakob disease.

It's important to note that not all bacteria, viruses, fungi, parasites, or prions are pathogenic. Many of these organisms are harmless or even beneficial to humans.