 Frenkel and those before him believed that all "Pneumocystis" were protozoans, but soon afterwards evidence began accumulating that "Pneumocystis" was a fungal genus.Recent studies show it to be an unusual, in some ways a primitive genus of Ascomycota, related to a group of yeasts.Every tested primate, including humans, appears to have its own type of "Pneumocystis" that is incapable of cross-infecting other host species and has co-evolved with each species.