In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host.
[citation needed] The virulence factors of bacteria are typically proteins or other molecules that are synthesized by enzymes.
Therefore, strategies to combat certain bacterial infections by targeting these specific virulence factors and mobile genetic elements have been proposed.
Some viral virulence factors confer ability to replicate during the defensive inflammation responses of the host such as during virus-induced fever.
Extremely virulent strains can eventually evolve by mutation and natural selection within the virus population inside a host.
The term "neurovirulent" is used for viruses such as rabies and herpes simplex which can invade the nervous system and cause disease there.