Fungi are microorganisms that exist in nature as one-celled yeasts or as branching filamentous molds (also may be spelled "moulds"). There are more than 50,000 species of fungi in the environment, but less than 200 species are associated with human disease. Of these, only about 20 to 25 species are common causes of infection. Fungal tests detect infections and sometimes identify the fungus and help guide treatment.