Microbiology
Pathogen
A microorganism or other agent that causes disease in a host. Includes bacteria, viruses, fungi, parasites, and prions.
Biological Context
Pathogens and hosts engage in evolutionary arms races—pathogens evolve to evade immune defenses, hosts evolve resistance. Pathogen virulence (harm caused) is subject to evolution: too virulent pathogens may kill hosts before spreading, selecting for moderate virulence in many cases.