Vaginal Yeast Infections
Most women will have a vaginal yeast infection at some point in their lives. The main species of yeast, Candida albicans, lives naturally and harmlessly in the vagina (yeast is a type of fungus; see also Differences Among Germs). It only causes symptoms when it grows too heavily. This commonly occurs after antibiotic treatment for any other infection, since the drug kills off other harmless vaginal bacteria which keep Candida in check. It is also common in uncontrolled Diabetes, advanced HIV disease, and among women taking medications which suppress the immune system (steroids and others). Reported factors such as douching, tight clothing, deodorants, etc. have not been proved. Yeast is not an STD.
Symptoms are mainly an itchy vaginal discharge, and/or irritation of the labia. Diagnosis is made by seeing the fungus under a microscope, which finds it 50% of the time. Since not every medical setting has one available, and since the test is only 50% sensitive, many clinicians simply treat, especially if a woman had taken antibiotics recently, has uncontrolled diabetes, or has had prior such infections. Medical texts discourage this, since it risks missing other causes. A good compromise would be testing the vaginal pH by a simple strip; if 4.5 or less, other causes are less likely.
There are other tests that can be done, such as culture, or PCR. These are expensive, and may take time. Point-of-care tests (done then and there on-site) may not be so accurate.
Treating partners is not useful, because women get yeast infections from their own naturally microbes; as noted, it is not an STD. There are no complications from untreated vaginal yeast infections. But they can be awfully uncomfortable.
Other Yeast Infections
Yeast infections also commonly occur on the skin: diaper rash in babies, and “Intertrigo” in areas where skin touches skin (e.g. the groin, under the breasts, the armpits, and under a paunch in the very obese). Babies and occasional people using steroid asthma inhalers can get yeast in the mouth (called “Thrush”); also persons with advanced HIV Disease. Men can get yeast Balanitis on the head of the penis. People who are severely immunocompromised can get catastrophic Candida in virtually any organ and even the bloodstream, but that has nothing to do with skin or genital infections.