The classification of clinically important yeasts and diagnosis of the role they play in the infectious process are dependent on culture of the organism from clinical material and/or on visualizing structures consistent with their morphology in diseased tissues. However, these methods may provide ambiguous results. Cultures may be positive or negative (that is, the organisms may or may not grow in the culture from a sample). Especially when yeasts are isolated from specimens such as sputum and skin, several problems arise in determining the clinical relevance of these findings. Healthy humans frequently harbor yeasts in the absence of disease as part of the indigenous flora of the oral mucosa; and women, under certain metabolic and physiological conditions, may be culture-positive for yeasts in vaginal swab samples. In addition, the lower gastrointestinal tract serves as a reservoir for yeasts and these organisms may transiently colonize the skin surrounding the tract exit.