How Does a Yeast Infection Occur?

A Delicate Balance

Yeast and bacteria live in a delicate balance in the vagina. Taking antibiotics can kill the good bacteria that keep yeast in check. Many women experience vaginal yeast infections after taking a course of antibiotics. Some topical antibiotics can also have this effect, particularly on skin surface infections. If you notice you always get a yeast infection after taking antibiotics, ask your doctor to prescribe you a yeast infection treatment, just in case.
Regular douching also washes away beneficial vaginal bacteria and can make women prone to an overgrowth of yeast. For this and other reasons, it is never a good idea to douche unless directed by your doctor.

Causes

Yeast flourishes in warm, moist conditions, like those in the genitals. Wearing pantyhose, a wet bathing suit or sweaty workout clothes for long periods of time provides the perfect moist habitat for yeast to thrive in. Keeping your skin clean and dry, and removing dirty, wet clothes as soon as possible will go a long way toward preventing an infection.
You might be surprised to learn that favorable conditions also occur as a result of high blood sugar. Uncontrolled diabetes can trigger regular, persistent yeast infections because high blood sugar causes glucose-rich vaginal secretions that yeast readily feed on.

the Vaginal Environment

For some women, yeast infections come on any time there is a change in the vaginal environment. These changes can range from natural hormonal changes during the menstrual cycle, an irritation caused by a certain fabric or dye, pregnancy, oral contraceptives or a new sexual partner. As the vagina experiences a hormonal change or becomes irritated, vaginal secretions can increase or decrease, changing the amount of moisture in the vagina as well as the yeast-to-bacteria ratio.
Other causes of change to the vaginal environment are poor hygiene, use of perfumed vaginal cleansing products or sprays, soap or detergent allergies, poorly ventilated clothing or underwear, and menopause or medication side effects.

Weakened Immune System

Your body's immune system usually kicks in to help fight burgeoning infections, and sometimes this can be enough to stop them in their tracks. For people who suffer from compromised immune systems, that might not be the case. People with immune system disorders like HIV, people on immunosuppressant drugs, or people with certain kinds of cancer can suffer from frequent yeast infections. To the same extent, people who experience regular fatigue, stress, poor diet or lack of exercise may also find themselves prone to yeast infections due to weakened immune systems.
Most women will get at least one yeast infection in their lifetimes. If yeast infections become recurring or persistent, identifying the potential cause(s) of the infection can go a long way toward future prevention.