About 75% of women will get at least one vaginal yeast infection in their lifetime, and up to half will get a second one. The reason comes down to a simple imbalance: a type of yeast called Candida already lives in the vagina in small amounts, and when the conditions that keep it in check shift, it multiplies rapidly. Understanding what those conditions are, and what disrupts them, explains why yeast infections are so common.
How the Vagina Normally Keeps Yeast Under Control
A healthy vagina isn’t sterile. It hosts a community of microorganisms, and the dominant residents are bacteria from the Lactobacillus family. These bacteria produce lactic acid, which keeps the vaginal environment acidic (typically a pH between 3.8 and 4.5). That acidity is hostile to yeast. As long as Lactobacillus populations are thriving, Candida stays in small, harmless numbers.
Not all Lactobacillus species are equally protective. Research published in the American Journal of Obstetrics & Gynecology found that one species, L. crispatus, produces significantly more lactic acid than another common species, L. iners, and inhibits Candida growth more effectively. Women whose vaginal microbiome is dominated by L. crispatus appear to be better protected against yeast colonization than those dominated by L. iners. This is one reason two women with seemingly identical habits can have very different susceptibility to yeast infections.
The vaginal lining itself also plays an active role. Epithelial cells lining the vaginal wall can directly inhibit Candida growth by recognizing the yeast through immune receptors on their surface and triggering a defensive response. Interestingly, this protection relies primarily on the innate immune system rather than the adaptive immune system. That’s why women with HIV, who have compromised adaptive immunity, don’t have higher rates of vaginal yeast infections even though they’re more vulnerable to oral and esophageal yeast infections.
Hormonal Changes Fuel Yeast Growth
Estrogen is one of the biggest behind-the-scenes players in yeast infections. Higher estrogen levels cause vaginal cells to store more glycogen, a form of sugar that breaks down into glucose. That glucose feeds both Lactobacillus and Candida. Under normal circumstances the extra fuel helps Lactobacillus keep the environment acidic enough to suppress yeast. But when something else disrupts the bacterial balance, all that available sugar becomes a feast for Candida instead.
This is why yeast infections cluster around specific hormonal events. Pregnancy raises estrogen dramatically, making yeast infections more frequent during the second and third trimesters. Hormonal birth control, particularly high-estrogen formulations, can have a similar effect. Many women notice infections in the days just before their period, when progesterone is high and the vaginal environment shifts. Menopause, which drops estrogen sharply, can also change the vaginal microbiome in ways that allow yeast to take hold, though the mechanism differs.
Antibiotics Wipe Out Protective Bacteria
Antibiotics are one of the most well-established triggers for yeast infections. They’re designed to kill bacteria, but they don’t distinguish between harmful bacteria causing your sinus infection and the beneficial Lactobacillus keeping your vaginal pH low. When Lactobacillus populations crash, the acidic environment weakens, and Candida seizes the opportunity to multiply.
Broad-spectrum antibiotics (the kind prescribed for common infections like urinary tract infections, strep throat, or respiratory infections) carry the highest risk because they target the widest range of bacteria. The yeast infection often shows up during or shortly after a course of antibiotics, which can feel like trading one problem for another.
High Blood Sugar Creates a Yeast-Friendly Environment
Women with diabetes, particularly poorly controlled diabetes, are significantly more prone to yeast infections. The CDC notes that when blood sugar runs high, excess sugar can be released through bodily fluids including vaginal secretions. That sugar-rich environment directly encourages yeast and bacteria to grow.
This doesn’t only apply to women with a diabetes diagnosis. Blood sugar spikes from prediabetes or insulin resistance can have similar effects. Women who notice frequent yeast infections without an obvious cause may want to have their blood sugar levels checked, as recurrent infections are sometimes one of the earliest signs of glucose regulation problems.
Moisture and Warmth Help Yeast Thrive
Candida grows best in warm, moist environments with limited airflow, which is exactly what tight, non-breathable clothing creates. The Cleveland Clinic recommends 100% cotton underwear because it wicks moisture away from the skin, depriving yeast and bacteria of the damp conditions they need. Synthetic fabrics trap heat and sweat against the vulva, and even underwear made from synthetic fabric with a small cotton crotch panel doesn’t fully compensate.
The same principle explains why yeast infections are more common in summer months or among women who stay in wet swimsuits or sweaty workout clothes for extended periods. The fix is straightforward: change out of wet or sweaty clothing promptly, choose breathable fabrics when possible, and avoid sitting in damp conditions longer than necessary.
Other Common Triggers
Several other factors can tip the balance toward yeast overgrowth. Douching, scented vaginal products, and harsh soaps disrupt the vaginal microbiome by altering pH or killing protective bacteria. A weakened immune system from stress, lack of sleep, or immunosuppressive medications reduces the body’s ability to keep Candida in check. Some women find that certain lubricants or spermicides irritate the vaginal lining enough to create vulnerability.
Genetics also play a role. Research has identified specific variations in immune receptor genes that are associated with higher rates of recurrent yeast infections. Some women’s immune cells are simply less efficient at recognizing and responding to Candida overgrowth, which helps explain why certain women get infections repeatedly while others rarely or never do.
When Yeast Infections Keep Coming Back
For 5 to 10% of women, yeast infections become a recurring problem, defined as four or more episodes in a single year. Recurrent infections aren’t just “bad luck” repeated. They often reflect an underlying factor that hasn’t been addressed: undiagnosed blood sugar issues, a chronic microbiome imbalance, a genetic predisposition in immune function, or ongoing exposure to a trigger like a particular medication.
Women with recurrent infections typically need a different treatment approach than the standard single-dose antifungal used for a one-off infection. Longer courses of treatment or maintenance regimens are common. Identifying and addressing the root cause, whether it’s switching birth control, managing blood sugar, or rebuilding vaginal bacteria, matters more than treating each episode individually.
How to Tell It’s Actually a Yeast Infection
One of the most common mistakes is assuming any vaginal discomfort is a yeast infection. Bacterial vaginosis (BV) causes similar irritation but requires completely different treatment. The discharge offers the clearest clue: yeast infections typically produce thick, white, odorless discharge, sometimes described as resembling cottage cheese, along with a white coating in and around the vagina. BV discharge tends to be grayish, thin or foamy, and has a noticeable fishy smell.
Studies have found that women who self-diagnose yeast infections are wrong roughly half the time. Using over-the-counter antifungal treatment for what turns out to be BV won’t help and may delay effective treatment. If the symptoms are new, unusual, or don’t resolve with standard antifungal treatment, getting tested gives a definitive answer and ensures you’re treating the right condition.