Cryptococcosis is an illness that affects a wide variety of mammals, including humans, with occasional cases also reported in birds, reptiles and amphibians. Two species, Cryptococcus neoformans and C. gattii, are responsible for most clinical cases. C. neoformans is an opportunistic human pathogen, and primarily affects people who are immunosuppressed; however, this does not seem to be the case for C. gattii in humans, or for either organism in animals. While C. neoformans and C. gattii are very common in some environments, most people and animals do not become ill after exposure. In a minority of cases, however, fungal infections become symptomatic in the respiratory tract, central nervous system (CNS) or other organs. Some infections are contained but not eliminated by the immune system, and can recur later in life. Cryptococcosis is sometimes fatal despite treatment.