Most of these fungi are free-living saprobes, existing in the environment in decaying vegetation, bird and animal excreta, humus and soil. Infections in humans follow inhalation or traumatic implantation and are not usually spread from man to man. In the sense that infection is accidental, these fungi are opportunistic. Molds are composed of tubular filaments called hyphae. These hyphae intertwine to form the mycelium, that is visible on natural substrates and on culture media at 25 C as the fungal colony.
Yeasts are oval, unicellular fungi that reproduce by budding.
Most are dimorphic. The parasitic form occurs in host tissues and on enriched media at 37 C and generally appear as yeast-like cells. The parasitic form can be converted to the saprobic form by changing the environmental conditions. The genus Candida comprises several dozen species of which at least eight have been reported to cause diseases in man. Candida albicans is the most important pathogen in the group.