Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, multiplies beyond what your body can keep in check. About 75% of women will experience at least one vaginal yeast infection in their lifetime, and 138 million women worldwide deal with recurrent infections every year. The triggers range from hormonal shifts and antibiotics to something as simple as sitting in a wet bathing suit too long.
Candida Already Lives in Your Body
The fungus behind most yeast infections isn’t something you “catch.” Candida albicans is part of the normal microbial community in the vagina, mouth, gut, and skin. In small numbers, it coexists peacefully with the bacteria that keep it in check. The problem starts when something disrupts that balance and allows Candida to shift from a harmless, rounded yeast form into an aggressive, thread-like form called hyphae. These elongated filaments can physically penetrate the cells lining the vaginal wall and release a toxin that damages tissue, causing the inflammation, itching, and discharge that define a yeast infection.
What keeps Candida under control in healthy conditions is largely the work of Lactobacillus bacteria. These bacteria dominate a healthy vaginal microbiome and produce lactic acid, hydrogen peroxide, and other compounds that maintain a vaginal pH between 4 and 4.5. At that acidity level, Candida’s ability to form those invasive filaments is limited. Anything that reduces Lactobacillus populations or raises vaginal pH opens the door for overgrowth.
Antibiotics Are the Most Common Trigger
Antibiotics kill bacteria. That’s their job. But they don’t distinguish between the harmful bacteria causing your sinus infection and the beneficial Lactobacillus keeping Candida in check. A course of broad-spectrum antibiotics can wipe out enough protective bacteria to let Candida multiply rapidly. This is why yeast infections so often follow a round of antibiotics for an unrelated condition. The broader the antibiotic and the longer you take it, the higher the risk.
Hormones Play a Direct Role
Estrogen is a specific trigger for Candida’s shift into its aggressive form. This explains why yeast infections cluster around certain life stages and hormonal events. Pregnancy dramatically raises estrogen levels, making yeast infections significantly more common during the second and third trimesters. Birth control pills that contain higher doses of estrogen carry a similar risk. Hormone replacement therapy can have the same effect.
The menstrual cycle matters too. Many women notice yeast infections tend to appear in the days just before their period, when hormone levels shift. The 25 to 34 age group has the highest prevalence of recurrent infections globally, at about 9%, likely reflecting the intersection of peak reproductive hormones and other risk factors.
A Weakened Immune System
Your immune system is constantly surveilling Candida and keeping its population small. When immune function drops, that surveillance falters. People living with HIV are at higher risk, particularly when CD4 cell counts fall below 200. Immunosuppressive medications taken after organ transplants or for autoimmune conditions have a similar effect. Chronic stress and sleep deprivation also suppress immune responses enough to tip the balance.
Diabetes is one of the most significant risk factors. Poorly controlled blood sugar creates an environment where Candida thrives, both because elevated glucose feeds the fungus directly and because high blood sugar impairs immune cell function. If you’re getting frequent yeast infections and haven’t been screened for diabetes, that’s worth bringing up with your doctor.
Moisture and Clothing Choices
Candida grows fastest in warm, dark, moist conditions. Sitting in a wet bathing suit or sweaty workout clothes for hours creates exactly that environment. The longer moisture stays trapped against the skin, the more opportunity Candida has to multiply beyond normal levels.
Tight, non-breathable fabrics compound the problem. Synthetic underwear and snug leggings trap heat and moisture against the vulva, while cotton underwear and looser clothing allow airflow and evaporation. This doesn’t mean wearing leggings once will cause an infection, but the habit of staying in damp, tight clothing regularly raises your baseline risk. Changing out of wet swimwear or gym clothes promptly is one of the simplest preventive steps you can take.
Douching and Scented Products
Douching disrupts the vaginal microbiome by physically washing away Lactobacillus bacteria and altering pH. Research published in Medical Mycology found that women who douched regularly (multiple times a day) were significantly more likely to harbor harder-to-treat, non-standard Candida species compared to women who douched less frequently. Even occasional douching after menstruation or sex can reduce the protective bacterial population enough to invite overgrowth.
Scented soaps, bubble baths, vaginal sprays, and scented tampons or pads can irritate vaginal tissue and shift the microbial environment in the same direction. The vagina is self-cleaning. Warm water on the external area is all that’s needed. Products marketed for vaginal freshness tend to create the very problems they claim to prevent.
Sex Can Be a Factor, but It’s Not an STI
Yeast infections are not classified as sexually transmitted infections because they commonly develop without any sexual contact at all. That said, sexual activity can play a role. Candida can pass between partners during vaginal sex, and transmission from a partner with a penis to a partner with a vagina is the most common direction. It’s much less common for someone with a vagina to transmit Candida to a partner with a penis.
Sex can also trigger yeast infections indirectly. Friction and micro-abrasions during intercourse can irritate vaginal tissue, and some lubricants or spermicides alter vaginal pH. New sexual partners may also introduce different microbial communities that temporarily disrupt the existing balance.
Diet and Blood Sugar
A diet very high in sugar and refined carbohydrates can raise blood glucose levels enough to promote Candida growth, even in people without diabetes. Candida feeds on sugar, and the vaginal tissue reflects what’s happening systemically. This doesn’t mean eating a cookie causes an infection, but a consistently high-sugar diet is a recognized contributing factor, particularly for women who get recurrent infections.
When Infections Keep Coming Back
Recurrent vulvovaginal candidiasis is defined as three or more symptomatic episodes within a single year, and it affects fewer than 5% of women. But within that group, the burden is substantial. Globally, 372 million women deal with recurrent yeast infections over the course of their lifetimes.
Recurrence often involves overlapping risk factors rather than a single cause. A woman on oral contraceptives who takes frequent antibiotics and has borderline blood sugar control faces a compounding risk that’s greater than any one factor alone. For women in this category, identifying and addressing the underlying drivers matters more than simply treating each individual episode. Sometimes the solution is switching contraceptive methods, improving blood sugar management, or adding a probiotic that supports Lactobacillus populations, rather than relying on antifungal treatment alone.