Many microorganisms have evolved to harmlessly colonize animals. The term ‘pathogen’ is used for an organism which causes disease in the host.
Some microorganisms can cause diseases in some individuals and not others. Such variations occur because of the presence or absence of virulence factors that can increase infectiousness or survivability, and/or variation in the host immune response.
The presentation of disease can also vary considerably based on the type of pathogen and its location in the host.