About 75% of women will have at least one vaginal yeast infection in their lifetime, and up to 45% will have two or more. Yeast infections happen when a fungus called Candida, which normally lives on your skin and mucous membranes in small amounts, multiplies beyond what your body can keep in check. Understanding what triggers that overgrowth can help you reduce your risk.
How Your Body Normally Keeps Yeast in Check
Your vaginal environment is home to protective bacteria, primarily Lactobacillus species, that act as a natural defense against Candida. These bacteria produce acids and other compounds that keep the vaginal pH low, making it inhospitable for yeast to multiply. They also release substances that prevent Candida from attaching to tissue and forming the thread-like structures it uses to burrow deeper into cells. Some Lactobacillus strains even produce enzymes that break down the yeast’s cell wall directly.
When something disrupts this bacterial population or changes the local environment, Candida gets the opportunity to grow unchecked. Nearly every cause of yeast infections traces back to one of a few core disruptions: killing off protective bacteria, feeding the yeast extra sugar, suppressing the immune system, or creating warm, moist conditions where yeast thrives.
Antibiotics
Antibiotics are one of the most common triggers. They work by killing bacteria, but they don’t distinguish between harmful bacteria and the protective Lactobacillus in your vaginal or gut microbiome. With those beneficial bacteria reduced, Candida faces less competition and can rapidly overgrow. This is why many women develop a yeast infection during or shortly after a course of antibiotics, regardless of the type prescribed. The longer and broader the antibiotic course, the greater the disruption to your microbial balance.
Hormones and Estrogen
Elevated estrogen levels are a major risk factor. Estrogen stimulates the cells lining the vagina to produce glycogen, a stored form of sugar. Candida albicans, the species responsible for most yeast infections, can feed directly on glycogen in a way that most other Candida species cannot. All 34 strains of C. albicans tested in one study were able to break down and use glycogen as fuel, while 26 non-albicans species could not.
This explains why yeast infections are more common during pregnancy, when estrogen levels surge. It also explains why high-estrogen oral contraceptives and hormone replacement therapy increase risk. In a study of postmenopausal women, those using estrogen therapy had significantly higher rates of Candida infection compared to those who did not. Beyond providing extra fuel for yeast, elevated estrogen also reduces the activity of immune cells in vaginal tissue and weakens the antifungal defenses of the cells lining the vaginal wall.
Blood Sugar and Diabetes
High blood sugar creates a sugar-rich environment throughout the body, including in vaginal secretions and urine. For people with diabetes, the longer blood sugar stays above roughly 250 mg/dL, the more likely yeast will grow. Excess sugar that the body can’t use gets filtered out through urine, and that sugar-laden urine passing over genital tissue encourages Candida to flourish.
High glucose also helps Candida evade the immune system. In lab studies, yeast cells exposed to higher glucose concentrations were better at avoiding detection and destruction by immune proteins. Certain diabetes medications called SGLT-2 inhibitors, which work by flushing extra sugar out through urine, can further increase yeast infection risk for the same reason. Reducing daily carbohydrate intake to around 150 grams, rather than 250 or more, can lower the amount of sugar excreted in urine and reduce the frequency of infections.
Weakened Immune System
Your immune system normally keeps Candida populations small. When that immune surveillance is compromised, yeast can proliferate. The CDC lists several conditions and treatments that raise risk:
- HIV/AIDS: Oral and esophageal yeast infections are among the most common infections in people living with HIV, and vaginal yeast infections also occur more frequently.
- Cancer and chemotherapy: Both the disease and its treatment suppress immune function, making Candida overgrowth more likely.
- Corticosteroids: Oral or inhaled steroids, commonly used for asthma and autoimmune conditions, dampen the local and systemic immune response. Inhaled corticosteroids are a particularly well-known cause of oral thrush.
Clothing and Moisture
Candida thrives in warm, moist environments. Synthetic underwear made from nylon or spandex traps heat and moisture against your skin, creating ideal conditions for yeast to grow. Cotton, by contrast, is breathable and absorbent, wicking moisture away from the skin’s surface. Tight-fitting clothing, especially during exercise, compounds the problem by limiting airflow.
Changing out of sweaty workout clothes promptly, choosing moisture-wicking fabrics for exercise, and switching to cotton underwear for daily wear all help keep the area drier. Some people who deal with recurrent infections find that sleeping without underwear helps, giving the area a chance to air out overnight. Drying underwear on low heat can also reduce any yeast or bacteria that survive the wash cycle.
Hygiene Habits That Backfire
Aggressive cleaning can be just as problematic as not cleaning enough. Douching, scented soaps, and fragranced feminine products can strip away protective bacteria and alter vaginal pH, creating an opening for Candida. The vagina is self-cleaning, so warm water and, if desired, a mild unscented soap on external areas is sufficient. Harsh soaps that aren’t fully rinsed away can also irritate tissue, making it more vulnerable to infection.
Yeast Infections in Men
Men can develop yeast infections too, most commonly as candidal balanitis, an infection of the head of the penis. It occurs most often in uncircumcised men and children under age four, because the foreskin creates a warm, moist fold where Candida naturally accumulates. A condition called phimosis, where the foreskin can’t be fully retracted, increases risk further by making thorough cleaning difficult. Poor hygiene, using harsh soaps, or not rinsing and drying the area completely also contribute. The same systemic factors that affect women, such as diabetes, antibiotics, and immune suppression, raise risk in men as well.
Sexual Activity
Yeast infections are not classified as sexually transmitted infections, but sexual activity can play a role. Intercourse can introduce new microorganisms, cause minor tissue irritation, and shift the vaginal bacterial balance. Some lubricants and spermicides contain ingredients that disrupt the vaginal microbiome. Partners can also pass Candida back and forth, which is why recurrent infections sometimes improve when both partners are treated.
Multiple Factors Often Overlap
In many cases, yeast infections result from several triggers hitting at once. A woman taking antibiotics for a sinus infection while pregnant, or a person with diabetes who just finished a course of steroids, faces compounding risks. Recurrent yeast infections, defined as four or more episodes in a year, often involve an underlying factor like uncontrolled blood sugar, chronic immune suppression, or a shift toward non-albicans Candida species that are harder to treat with standard antifungal medications. If infections keep coming back, identifying and addressing the underlying trigger is usually more effective than treating each episode individually.