Several species of the genus Candida may cause infections in humans, but Candida albicans is the usual causative agent. Other species have come to prominence as causes of disease, especially in immunocompromised patients. These include C. tropicalis, C. pseudotropicalis, C. paratropicalis, C. krusei, C. guilliermondii, C. parapsilosis, C. glabrata, C. dubliniensis, C. inconspicua, C. lusitaniae, C. rugosa, and C. stellatoidea. These yeasts are round to oval vegetative cells that, under conditioned circumstances, produce pseudohyphae. Characteristically, C. albicans develops chlamydospore formation under stressful, controlled conditions, whereas other species of Candida do not exhibit such structures.

Candida species are highly prevalent in nature and are found predominantly in association with humans and other warm-blooded animals. C. albicans may be isolated from soil, but usually only at sites where human or animal contamination has occurred. One species, C. stellatoidea, has been isolated only from humans. Within the animal kingdom, C. albicans has been isolated from a variety of wild and domestic animals.

Likely, transmission of Candida involves direct contact with a colonized site. Oral thrush in neonates results from organisms that are acquired during passage through the birth canal or from colonized nipples of the mother or a nursing bottle. Although C. albicans has been isolated from the air around patients with cutaneous candidiasis, the extent of airborne transmission has not been established. Colonization with C. albicans occurs in most infants and children and is not associated with discernible illness. Receptor sites on epithelial cells of the mucosa permit adherence of the yeast form and the establishment of colonization.

Several functions of the immune system actively defend against this organism in healthy hosts. Secretory and humoral IgA antibodies are generated, as are specific anticandidal IgE, IgG, and IgM antibodies. The organism can activate an alternate complement pathway. Neutrophils, monocytes, and eosinophils can ingest and kill the yeast. The organism can induce the formation of suppressor and mitogen-stimulated lymphocytes and thereby produce a lymphokine that will kill it. Lactoferrin has anticandidal activity. Although mucosal surfaces are colonized easily, the normal skin is relatively resistant to colonization and infection with Candida species.

At high risk for acquiring infections from Candida species are patients with acquired and congenital immune deficiency disorders, cancer, certain endocrinopathic conditions, diabetes mellitus, burns, trauma, and malnutrition; organ transplant recipients; individuals receiving immunosuppressive drugs such as corticosteroids; and very low birth-weight infants. Healthy individuals may be at increased risk for candidiasis of the mucous membranes during infancy, pregnancy, and old age. Invasive disseminated candidiasis has been encountered in 7% of children infected with the human immunodeficiency virus and in 11% of recipients of bone marrow transplants. Surveillance studies show the annual incidence of 8 per 100,000 population in the United States. The highest incidence (75 per 100,000) occurs among infants 1 year of age and younger. A significant shift has occurred in the epidemiology of hematogenous candidiasis caused by different Candida species. A relative decrease in C. albicans and C. tropicalis is associated with an increase in C. krusei and C. glabrata. This shift has been attributed in part to the use of fluconazole. The invasive form of candidiasis may be found in one-third of children with malignant tumors and febrile-neutropenic episodes that are not responsive to antibiotics. Approximately 10% of preterm infants with birth weights of less than 1,000 grams have infections caused by Candida species acquired in the neonatal intensive care unit and one-third of these infections are fatal.

The initial step in infection is adherence of the yeast form of Candida to an epithelial cell surface. The adherent blastospore then develops a filamentous or pseudohyphal form, and the organism becomes invasive. In the case of mucous membranes, infection such as thrush develops as an adherent pseudomembrane composed of epithelial cells, leukocytes, keratin, food debris, and both yeast and pseudohyphal forms of Candida. Mucosal lesions may progress to well-demarcated ulcers, especially in the intestinal tract, with a base of granulation tissue covered by a fibrinous exudate and granulocytes. Organisms may invade blood vessels, may become bloodborne, and may disseminate to any organ in the body in immunosuppressed patients. From the mucosal portal of entry, a systemic disease evolves, with the kidneys, lungs, liver, brain, and spleen affected most frequently. The gut is the major site for entry of organisms in candidemia. In systemic disease, a pyogenic response occurs, with microabscess formation. Granulomatous reactions occur infrequently.

The clinical features of candidiasis may be considered in three categories: those associated with mucous membranes, those associated with the skin, and those in which systemic invasion has occurred.