Infection
 Among the vast varieties of microorganisms, relatively few cause disease in otherwise healthy individuals. Infectious disease results from the interplay between those few pathogens and the defenses of the hosts they infect. The appearance and severity of disease resulting from any pathogen, depends upon the ability of that pathogen to damage the host as well as the ability of the host to resist the pathogen. However a host's immune system can also cause damage to the host itself in an attempt to control the infection. Clinicians therefore classify infectious microorganisms or microbes according to the status of host defenses - either as primary pathogens or as opportunistic pathogens:

Q: What does the severity of a disease resulting from a pathogen depend on?
ability of that pathogen to damage the host