Dear Doctor:

I had a yeast infection, and even though I did everything the doctor told me to, it has come back twice. What causes a yeast infection? How do I keep from getting another one?

Dear Reader: About three-quarters of all women are regrettably familiar with the unique torment of a vaginal yeast infection. Also known as vaginal candidiasis, it’s a condition that occurs due to an overgrowth of a species of Candida fungus, most often Candida albicans. The result is irritation, discharge, odor and a nonstop, maddening itch.

An overgrowth of Candida can occur when something disrupts the pH balance within the vagina. This prevents certain bacteria naturally present in the vagina, including Lactobacillus, from successfully keeping the fungal growth in check.

Despite the discomfort, a yeast infection isn’t considered to be serious. Over-the-counter topical treatments like Monistat, which are used from one to three days, are usually quite effective. (Please note that many of these topicals are oil-based, which can weaken a latex condom and a diaphragm.) In more serious or persistent cases, oral fluconazole, a prescription medication, may be required.

When over-the-counter treatments don’t cure the infection, or when symptoms recur within eight weeks, it’s important to seek a medical opinion. Several other conditions, including a vaginal infection known as bacterial vaginosis, and trichomoniasis, a common sexually transmitted disease, may have similar symptoms to a yeast infection, but require different treatments.

Causes of a yeast infection include the use of oral antibiotics, which affect the balance of bacterial colonies throughout the body. Spermicides can adversely affect the vaginal environment, as can the prolonged use of tampons.

Glycerin, which is present in many lubricants, can be a contributing factor in yeast infections. The mouths of up to half of adults contain Candida albicans, which has led researchers at the University of Michigan, Ann Arbor to link oral sex to an increased risk of developing a yeast infection. The hormonal changes that occur during pregnancy or menopause, a weakened immune system and high blood sugar can also be contributing factors.

To prevent yeast infections, remember that fungi thrive on warmth and moisture. Wear breathable underwear, and avoid prolonged time in a damp bathing suit, sweaty gym clothes or tight pants. If you use panty liners, change them frequently.

Dry off thoroughly after bathing; don’t use douches or sprays, which adversely alter the chemical environment of the vagina; and steer clear of scented tampons, pads or other feminine products.

Unfortunately, about half of women will go on to have a second yeast infection, and an unlucky 5% are faced with recurrent vaginal candidiasis, which is four or more yeast infections per year.

To reiterate: Whenever a yeast infection persists despite treatment, or when one recurs within a few weeks, contact your health care provider.