Microorganisms (pathogens) can lead to disease. The pathogens invade humans (or other living organisms), and their growth and reproduction can cause disease. For example, a virus can infect a human and spread throughout the body. With a vaccine or other treatment, the virus is removed and the disease along with it.

This is also the current dominating view of mental disorders where a pathogen/latent disorder/brain aberration cause the symptoms seen.