From an ecological point of view, pathogens and autochthonous commensals are species that enhance their evolutionary fitness by using an animal host in any part of their life cycle. Conversely, growth on or in humans is evolutionary spill-over for opportunists and allochthonous commensals, decreasing the species' fitness. Strategic use of animals includes decrease of virulence over evolutionary time, such as to guarantee optimal condition of the host for dispersal and carriage to nutrient sources. Pathogens are able to overcome the actions of innate immune cells, and are limited or dormant with functioning acquired cellular immunity. Opportunists are poorly adapted, and hence are more virulent and less specific in their pathology. The great majority of opportunistic infections is quickly resolved by phagocytic action, but occasionally chronic infections are noted, where acquired immunity nevertheless remains insignificant. The number of opportunistic species is very small compared to the number of extant fungi, and therefore strategic and coincidental virulence factors must be of a quite specific nature. We aim to understand the role of such factors by revealing counterparts in the natural ecological niches of recurrent opportunists, such as Scedosporium species and black yeasts with their relatives. As an example, a possible clue to the high frequency of Asian cerebral infections by Chaetothyrialean fungi may their ablity of alkylbenzene assimilation.