Among the many organisms that live in the human mouth and digestive tract is the yeast Candida albicans, which under normal circumstances lives in 80 percent of the human population with no harmful effects. Overgrowth, however, results in candidiasis. Candidiasis, commonly called yeast infection or thrush, is a fungal infection of any of the Candida species, of which Candida albicans is probably the most common. Yeast organisms are always present in all people, but are usually prevented from "overgrowth" (uncontrolled multiplication resulting in symptoms) by naturally occurring microorganisms.In immunocompetent people, candidiasis can usually only be found in exposed and moist parts of the body, such as the oral cavity (oral thrush), the vagina (vaginal candidiasis or thrush), or folds of skin in the diaper area (diaper rash). Candidiasis is the most common cause of vaginal irritation or vaginitis.At least three quarters of all women will experience candidiasis at some point in their lives. The Candida albicans organism is found in the vaginas of almost all women and normally causes no problems. However, when it gets out of balance with the other "normal flora", such as lactobacilli (which can also be harmed by using douches), an overgrowth and symptoms can result. Pregnancy, the use of oral contraceptives and some antibiotics, and diabetes mellitus increase the risk of infection.The most common symptoms are itching and irritation of the vagina and/or vulva. A whitish or whitish-gray discharge may be present, sometimes resembling cottage cheese, and may have a "yeasty" smell like beer or baking bread. The normal smell is a lactic acid smell, like cheese or yogurt, due to lactobacilli, "good" bacteria that are also used to turn milk into cheese and yogurt.In immunocompromised patients, the candida infection can become systemic, causing much more serious disease.Candida albicans, a diploid sexual fungus (a form of yeast) is the causal agent of opportunistic infections in humans, the most common being oral and vaginal infections. Systemic fungal infections have emerged as important causes of morbidity and mortality in immunocompromised patients (e.g., AIDS, cancer chemotherapy, organ or bone marrow transplantation). In addition, hospital-related infections in patients not previously considered at risk (e.g. patients on an intensive care unit) have become a cause of major health concern.