Candida yeasts are commonly present in humans, and their growth is normally limited by the human immune system and by other microorganisms, such as bacteria occupying the same locations (niches) in the human body.
C. albicans was isolated from the vaginas of 19% of apparently healthy women, i.e., those that experienced few or no symptoms of infection. External use of detergents or douches or internal disturbances (hormonal or physiological) can perturb the normal vaginal flora, consisting of lactic acid bacteria, such as lactobacilli, and result in an overgrowth of Candida cells causing symptoms of infection, such as local inflammation. Pregnancy and the use of oral contraceptives have been reported as risk factors, while the roles of engaging in vaginal sex immediately and without cleansing after anal sex and using lubricants containing glycerin remain controversial. Diabetes mellitus and the use of anti-bacterial antibiotics are also linked to an increased incidence of yeast infections. Diet high in simple carbohydrates has been found to affect rates of oral candidiases, and hormone replacement therapy and infertility treatments may also be predisposing factors. Wearing wet swimwear for long periods of time is also believed to be a risk factor.
A weakened or undeveloped immune system or metabolic illnesses such as diabetes are significant predisposing factors of candidiasis. Diseases or conditions linked to candidiasis include HIV/AIDS, mononucleosis, cancer treatments, steroids, stress, and nutrient deficiency. Almost 15% of people with weakened immune systems develop a systemic illness caused by Candida species. In extreme cases, these superficial infections of the skin or mucous membranes may enter into the bloodstream and cause systemic Candida infections.
In penile candidiasis, the causes include sexual intercourse with an infected individual, low immunity, antibiotics, and diabetes. Male genital yeast infection is less common, and incidence of infection is only a fraction of that in women; however, yeast infection on the penis from direct contact via sexual intercourse with an infected partner is not uncommon.
Candida species are frequently part of the human body's normal oral and intestinal flora. Treatment with antibiotics can lead to eliminating the yeast's natural competitors for resources, and increase the severity of the condition. Higher prevalence of colonization of C. albicans was reported in young individuals with tongue piercing, in comparison to non-tongue-pierced matched individuals. In the western hemisphere approximately 75% of females are affected at some time in their life.