Facultative pathogens are pathogens capable of causing disease in only certain circumstances. Due to their wide range of lifestyles, it is possible for facultative pathogens to live in a wide range of environmental niches. It is difficult to define the boundary between an opportunistic pathogen and a facultative pathogen. Most pathogens are thought to be facultative.
The flexibility of facultative pathogens means they have larger genomes to allow them to survive in many different environmental conditions.
Not all pathogens are equally virulent, and some may not have certain virulence factors.