A female patient is given an antibiotic for bronchitis. The bronchitis improves but she returns a short time later complaining of a vaginal discharge. Coincidence? Probably not.

Each of us has trillions of microbes living in us and on us. Usually these microbes are harmless, and indeed many are helpful. Most of these microbes are bacteria. Some are yeast cells, which are actually a one-celled fungus. These yeast cells usually do not cause any problems, unless something disrupts this microscopic world.

Antibiotics frequently are the cause of such a disruption. For example, if a patient with bronchitis is given an antibiotic, the antibiotic will hopefully kill the bacteria that are causing the bronchitis. However, it may also kill some of the bacteria that are keeping the yeast cells in check. Since antibiotics have no effect on yeast cells, the yeast cells are free to multiply uncontrollably, often leading to problems such as a vaginal yeast infection.

The resultant yeast infection will cause vaginal itching, burning and a very characteristic thick white discharge that looks much like cottage cheese. The diagnosis can often be made by examination alone but if necessary can be confirmed by examining the discharge under a microscope or by doing a culture. There are many over-the-counter treatments available that are effective. Prescription medications can be given for resistant cases.

Other possible disrupting factors include poorly controlled diabetes, pregnancy or problems with one’s immune system.

“Probiotics,” which are capsules containing live bacteria, and yogurt have been advocated by some for use in vaginal yeast infections, but it is not clear that they are actually helpful.

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Yeast infections can also affect other parts of the body. Moist, warm areas are especially susceptible. Many severe cases of diaper rash are caused by yeast, since the area under the diaper — despite best efforts — is often warm and moist. Likewise, women with large pendulous breasts will often develop yeast infections where the breasts continuously rub against the chest wall.

The mouth is also a frequent site of yeast infections. This is common in people who wear dentures or who have weakened immune systems due to such problems as cancer, HIV or diabetes.

Not all yeast cells are harmful. Yeast is used to make bread and cheese and to produce beer and wine. It's almost as if they are paying us back for all the trouble they cause.