Candida species are fungi that cause important human diseases collectively referred to as candidiasis. The diseases are usually endogenous in origin, meaning that the source of the pathogen is within the host. Candida albicans is a common, usually harmless inhabitant of mucosal surfaces of the oral cavity, gut, and vaginal canal along with a variety of other microorganisms that are collectively called the normal flora. Most often, candidiasis occurs in the immunocompromised host; cancer, transplant, diabetic, AIDS, and surgical patients as well as premature infants are especially susceptible to infections.

The reason for susceptibility varies. For the AIDS patient, a depletion in T-lymphocyte numbers triggers mucosal disease. The premature infant may develop candidiasis because of an underdeveloped immune system or because bacteria that are part of the normal flora of the human oral cavity or gut are not as protective. It is believed that competition among members of the normal flora of mucosal surfaces (oral cavity, vaginal system, gut) protects the host against the transgression of C. albicans. Consequently, if the competing bacteria at these locations are reduced in number by the prolonged use of antibiotics, an overgrowth of C. albicans can result in disease.