Cryptococcus neoformans is an encapsulated basidiomycetous yeast belonging to the class Tremellomycetes and an obligate aerobe that can live in both plants and animals. Its teleomorph is a filamentous fungus, formerly referred to Filobasidiella neoformans. In its yeast state, it is often found in bird excrement. It has remarkable genomic plasticity and genetic variability between its strains, making treatment of the disease it causes difficult. Cryptococcus neoformans causes disease primarily in immunocompromised hosts, such as HIV or cancer patients. In addition, it has been shown to cause disease in apparently immunocompetent hosts, especially in developed countries.