When considering pathogens,<unk>  can have varying descriptions. For example, in the case of Salmonella,<unk>  is used to describe the "ability of a pathogen to circulate and cause disease in a particular<unk>  population." Another usage of<unk> , still considering the case of Salmonella, refers to the evolution of a pathogen such that it can infect, cause disease, and circulate in another<unk>  species.
Host adaptation
Microbial synergy
Antigenic variation
Sex selection
Cancer selection
Viral interference
Functional specialization 
Immune tolerance