Yeast organisms are always present in all people, but are usually prevented from “overgrowth” (uncontrolled multiplication resulting in symptoms) by other naturally occurring microorganisms.

In a study of 1009 women in New Zealand, the fungus, Candida albicans, was isolated from the vaginas of 19% of apparently healthy women. Carriers experienced few or no symptoms. However, external use of irritants (such as some detergents or douches) or internal disturbances ((hormonal or physiological) can perturb the normal flora, constituting lactic acid bacteria, such as lactobacilli, and an overgrowth of yeast can result in noticeable symptoms. Pregnancy, the use of oral contraceptives, engaging in vaginal sex immediately and without cleansing after anal sex, and using lubricants containing glycerine have been found to be causally related to yeast infections. Diabetes mellitus and the use of antibiotics are also linked to an increased incidence of yeast infections. Candidiasis can be sexually transmitted between partners. Diet has been found to be the cause in some animals. Hormone Replacement Therapy and infertility treatments may also be predisposing factors.