Yeast infections happen when a fungus called Candida, which normally lives in the vagina in small amounts, grows out of control. About three out of four women will experience at least one vaginal yeast infection in their lifetime. The trigger isn’t a single cause but a shift in the vaginal environment that lets yeast multiply faster than the body can keep it in check.
The Balance That Keeps Yeast in Check
Your vagina naturally contains both yeast and bacteria in a carefully maintained balance. Beneficial bacteria, primarily Lactobacillus species, keep the vaginal environment slightly acidic. This acidity discourages yeast from multiplying. Lactobacillus bacteria also physically block yeast from attaching to the vaginal walls, essentially competing for the same real estate and winning.
When something disrupts this balance, Candida gets an opening. The yeast shifts from its harmless round form into an aggressive, thread-like form that can burrow into vaginal tissue, triggering inflammation, itching, and the thick white discharge most women recognize. The exact tipping point varies from person to person, which is why some women seem prone to yeast infections while others rarely get them.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics are the single most well-known cause of yeast infections. These medications, commonly prescribed for sinus infections, urinary tract infections, and bronchitis, don’t distinguish between harmful bacteria and the beneficial Lactobacillus in your vagina. When Lactobacillus populations drop, the vagina becomes less acidic, and yeast that was previously held in check begins to multiply freely.
Not every course of antibiotics leads to a yeast infection, but the risk rises with longer courses and broader-spectrum drugs. If you’ve noticed a pattern of yeast infections after taking antibiotics, that connection is well established and worth mentioning to your prescriber.
How Hormones Fuel Yeast Growth
Estrogen plays a central role in shaping the vaginal environment. It drives vaginal cells to produce glycogen, a sugar that feeds Lactobacillus bacteria but also serves as fuel for yeast. When estrogen levels rise, glycogen production increases, and Candida has more to feed on.
This explains why yeast infections cluster around specific hormonal events. Pregnancy raises estrogen significantly, making yeast infections more common throughout all three trimesters. Hormonal birth control, particularly higher-estrogen formulations, can have a similar effect. Some women notice infections at certain points in their menstrual cycle, when estrogen naturally peaks.
On the other end of the spectrum, the estrogen drop after menopause changes the vaginal environment in a different way. Lactobacillus populations decline, vaginal pH rises, and the microbial community shifts toward a more diverse mix that’s less protective against infections overall. So both high and low estrogen states can create vulnerability, though through different mechanisms.
High Blood Sugar Creates a Breeding Ground
Women with diabetes face a notably higher risk of yeast infections, particularly when blood sugar is poorly controlled. When glucose levels run high, excess sugar shows up in vaginal secretions and urine, essentially feeding Candida directly. The yeast thrives in this sugar-rich environment and multiplies faster than the body’s defenses can respond.
This risk isn’t limited to women with a diabetes diagnosis. Any condition or medication that raises blood sugar, including steroid medications like prednisone, can have the same effect. If you’re getting frequent yeast infections without an obvious cause, it’s worth having your blood sugar checked.
Clothing, Moisture, and Everyday Habits
Yeast grows best in warm, moist environments. Clothing choices that trap heat and moisture against the vulva can tip conditions in yeast’s favor. Tight synthetic fabrics, workout leggings worn for hours after exercise, and wet swimsuits all create the kind of environment Candida prefers. Cotton underwear is the most breathable option and wicks moisture away from the skin. Underwear marketed as having a “cotton crotch panel” in otherwise synthetic fabric doesn’t offer the same protection, according to Cleveland Clinic guidance.
Douching and scented vaginal products can also disrupt the microbial balance. These products alter vaginal pH and can reduce Lactobacillus populations, opening the door for yeast overgrowth. The vagina is self-cleaning, and external washing with mild, unscented soap is all that’s needed.
Can You Get a Yeast Infection From Sex?
Yeast infections are not classified as sexually transmitted infections, because you can develop one without any sexual contact. But sex can play a role. You can get a yeast infection from a sexual partner during vaginal, oral, or anal sex. The Office on Women’s Health notes that about 15% of men develop an itchy rash on the penis after unprotected sex with a woman who has an active yeast infection.
If your sexual partner is a woman, she may also be at risk and should be tested if she develops symptoms. Condoms and dental dams can reduce the chance of passing yeast back and forth. Partner treatment isn’t always necessary, but it’s worth considering if infections keep returning.
A Weakened Immune System Raises Risk
Your immune system is what normally keeps Candida from transitioning into its aggressive form. Anything that suppresses immune function, whether it’s HIV, chemotherapy, organ transplant medications, or chronic stress, can reduce the body’s ability to control yeast populations. Women who are immunocompromised tend to get more frequent and more severe yeast infections, and these infections can be harder to treat.
When Infections Keep Coming Back
Most women deal with an occasional yeast infection that clears with treatment. But fewer than 5% of women develop recurrent vulvovaginal candidiasis, defined as three or more symptomatic infections within a single year. Recurrent infections can signal an underlying issue like uncontrolled blood sugar, an immune deficiency, or a non-albicans Candida species that doesn’t respond to standard treatments.
Recurrent infections also sometimes happen without any identifiable risk factor. The interplay between Candida, the immune system, and the vaginal microbiome is complex, and researchers still don’t fully understand why some women’s bodies repeatedly fail to keep yeast in check. What is clear is that recurrent infections typically require a different, longer treatment approach than a one-time episode, so getting an accurate diagnosis matters more with each recurrence.