Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, grows out of control. About 75% of women will experience at least one yeast infection in their lifetime, and roughly 138 million women worldwide deal with recurrent infections each year. The trigger is almost always something that disrupts the vagina’s natural defenses, giving yeast the opening it needs to multiply.
The Balance That Keeps Yeast in Check
A healthy vagina hosts a community of bacteria dominated by Lactobacillus species. These bacteria produce lactic acid that keeps vaginal pH between 3.8 and 4.5, an acidic environment where Candida stays in check. Lactobacillus also competes directly with yeast for nutrients and attachment sites on vaginal tissue, and produces compounds that interfere with Candida’s ability to latch onto the vaginal walls.
When something reduces the Lactobacillus population or raises vaginal pH, Candida seizes the opportunity. The yeast shifts from a harmless bystander into an aggressive colonizer, producing the thick, clumpy white discharge, itching, and burning that characterize a yeast infection. Understanding what tips this balance is the key to understanding how infections start.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics, the kind prescribed for sinus infections, bronchitis, or urinary tract infections, don’t distinguish between harmful bacteria and helpful ones. They wipe out Lactobacillus right alongside the bacteria causing your illness. Without that protective bacterial layer, the vagina becomes less acidic, and yeast can multiply rapidly.
A yeast infection can develop during a course of antibiotics or shortly after finishing one. Not every woman who takes antibiotics will get a yeast infection, but women who are prone to them often notice the pattern. If you’ve experienced this before, your doctor may suggest a preventive antifungal alongside your antibiotic prescription.
Hormonal Shifts and Pregnancy
Estrogen plays a direct role in how much glycogen (a sugar that feeds vaginal bacteria) is available in vaginal tissue. When estrogen levels rise, as they do during pregnancy, in the second half of the menstrual cycle, or while taking hormonal birth control, glycogen levels increase. This can fuel yeast growth alongside the bacteria it’s supposed to support. Pregnant women are particularly susceptible because their estrogen levels remain elevated for months.
Hormonal shifts during perimenopause and menopause work differently. Dropping estrogen thins the vaginal lining and reduces Lactobacillus populations, which can also leave the door open for yeast overgrowth, especially when combined with other risk factors like diabetes.
High Blood Sugar Feeds the Yeast
Women with poorly controlled diabetes face a significantly higher risk of yeast infections. Chronic high blood sugar disrupts immune function, increases oxidative stress, and alters the vaginal microbiome. Elevated glucose in vaginal secretions essentially provides extra fuel for Candida to thrive on.
This connection goes beyond diagnosed diabetes. Even temporary blood sugar spikes from illness or medication can shift the vaginal environment. Some diabetes medications that work by flushing excess sugar through the urinary tract can themselves increase the risk of genital yeast infections, since that sugar passes through tissue where Candida lives.
Weakened Immune Defenses
Your immune system normally keeps Candida populations contained. Conditions that suppress immune function, including HIV, autoimmune disorders requiring immunosuppressive treatment, and chronic use of corticosteroids, all raise the likelihood of yeast infections. Even high stress or poor sleep can temporarily dampen your immune response enough to let yeast gain a foothold, though the effect is more modest than with medical immunosuppression.
Hygiene Products That Backfire
Douching is one of the clearest self-inflicted risk factors. It floods the vagina with water mixed with vinegar, baking soda, or iodine, stripping away the natural bacterial balance. Women who douche weekly are five times more likely to develop bacterial vaginosis, and the same disruption to vaginal flora increases yeast infection risk. Douching can also push existing infections deeper into the reproductive tract.
Scented tampons, pads, vaginal sprays, and powders pose a similar problem on a smaller scale. The chemicals in fragrances can irritate vaginal tissue and shift pH. The vagina is self-cleaning, so these products solve a problem that doesn’t exist while creating one that does.
Clothing, Moisture, and Warmth
Candida thrives in warm, moist environments. Tight-fitting synthetic underwear, wet swimsuits worn for hours, and non-breathable workout clothes all create conditions yeast loves. Cotton underwear and changing out of damp clothing promptly won’t guarantee you’ll avoid an infection, but they remove one contributing factor. The same logic applies to staying in sweaty gym clothes: the longer moisture sits against the skin, the more hospitable the environment becomes.
Sexual Activity
Yeast infections are not sexually transmitted infections, but sexual activity can contribute to them. Intercourse can introduce new bacteria, shift vaginal pH, and cause micro-irritation to vaginal tissue. Lubricants and spermicides containing glycerin or fragrances can also feed yeast or disrupt bacterial balance. Some women notice a pattern of infections after sex with a new partner as their vaginal flora adjusts.
What Recurrent Infections Look Like
Most women experience an occasional yeast infection and move on. But for a significant minority, infections keep returning. The CDC defines recurrent vulvovaginal candidiasis as three or more symptomatic episodes in a single year. This pattern affects fewer than 5% of women but has an outsized impact on quality of life. Globally, an estimated 372 million women experience recurrent infections over their lifetime.
Recurrent infections often involve the same underlying triggers on repeat: a cycle of antibiotics, ongoing blood sugar issues, or a chronically disrupted microbiome that never fully recovers between episodes. In some cases, the Candida strain involved is resistant to standard over-the-counter treatments, which is why infections that keep coming back warrant a clinical evaluation rather than another round of the same antifungal.
Recognizing the Symptoms
The hallmark of a yeast infection is thick, white, clumpy discharge often compared to cottage cheese. It typically has little to no odor, which helps distinguish it from bacterial vaginosis (fishy smell) or trichomoniasis (greenish, frothy discharge). Itching and irritation around the vulva are usually intense, and you may notice redness, swelling, or a burning sensation during urination or sex.
These symptoms overlap enough with other vaginal infections that self-diagnosis is wrong roughly half the time. If it’s your first infection, or if over-the-counter treatments aren’t clearing things up within a few days, getting tested ensures you’re treating the right problem. A simple swab can confirm whether Candida is actually the cause.