Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, multiplies beyond what the body can keep in check. An estimated 75% of women will experience at least one yeast infection in their lifetime, and 40% to 45% will have two or more. The triggers range from antibiotics and hormonal shifts to something as simple as wearing damp clothing too long.
What Happens Inside Your Body
Candida isn’t an invader. It already lives on your skin and mucous membranes, including the vaginal lining, without causing problems. The difference between harmless colonization and an actual infection comes down to shape-shifting: Candida cells can switch from a round yeast form into long, thread-like filaments called hyphae. These filaments physically burrow into the top layer of vaginal tissue and release a toxin that damages cells, triggering the inflammation, itching, and discharge you feel as a yeast infection.
What normally prevents this? Beneficial bacteria, primarily Lactobacillus species, act as a built-in defense system. They produce lactic acid that keeps vaginal pH between 4 and 4.5, an acidity level that suppresses Candida’s ability to form those invasive filaments. Lactobacilli also produce hydrogen peroxide, antimicrobial peptides, and compounds that strengthen the vaginal lining itself. When something disrupts this bacterial ecosystem, Candida gets the opening it needs.
Antibiotics Are the Most Common Trigger
Antibiotics are the single most recognized cause of yeast infections because they don’t distinguish between harmful bacteria and the protective Lactobacillus in your vagina. When Lactobacillus populations drop, vaginal pH rises, lactic acid production falls, and Candida can shift into its invasive form largely unopposed. This is why yeast infections so often follow a course of antibiotics for a completely unrelated problem, like a sinus infection or urinary tract infection.
Not every antibiotic course will trigger one, and some women are more susceptible than others. Broad-spectrum antibiotics, the kind prescribed when a doctor isn’t sure exactly which bacterium is causing an infection, carry the highest risk because they wipe out the widest range of bacteria.
Hormonal Changes and Estrogen
Estrogen increases the amount of glycogen (a sugar the body stores) in vaginal tissue, which feeds both Lactobacillus and Candida. During times when estrogen levels spike or fluctuate, the balance can tip toward Candida overgrowth. This is why yeast infections are more common during pregnancy, in the second half of the menstrual cycle, and in some women using hormonal birth control, particularly higher-dose estrogen formulations.
Before puberty and after menopause, when estrogen levels are naturally low, yeast infections are far less common. If you notice a pattern of infections tied to a specific point in your cycle or after starting a new contraceptive, that hormonal connection is likely playing a role.
High Blood Sugar Fuels Candida Growth
Candida thrives in high-sugar environments. When blood sugar is elevated, glucose concentrations in vaginal secretions rise as well, giving the fungus more fuel to multiply. This is why women with poorly controlled type 2 diabetes face a significantly higher risk of yeast infections, and why recurrent infections are sometimes the symptom that leads to a diabetes diagnosis in the first place.
You don’t need to have diabetes for this mechanism to matter. Diets very high in refined sugar can modestly raise glucose in vaginal tissues, though the effect is far less dramatic than in uncontrolled diabetes. The practical takeaway: if you’re getting frequent yeast infections and haven’t had your blood sugar checked recently, it’s worth doing.
Moisture, Clothing, and Hygiene Habits
Candida grows faster in warm, moist environments. Wearing wet swimsuits or sweaty workout clothes for extended periods creates exactly those conditions. Cotton underwear wicks moisture away from the skin, while synthetic fabrics trap it. Even underwear labeled as having a “cotton crotch panel” doesn’t fully protect you, because the surrounding synthetic material still limits airflow.
Douching, scented sprays, and fragranced soaps can also disrupt the vaginal microbiome by altering pH and killing off protective bacteria. The vagina is self-cleaning. Water and mild, unscented soap on the external vulva is all that’s needed. Anything introduced inside the vaginal canal risks upsetting the bacterial balance that keeps Candida in check.
Can You Get a Yeast Infection From Sex?
Yes, though yeast infections are not classified as sexually transmitted infections because they commonly occur in people who aren’t sexually active. Candida can be passed between partners during vaginal, oral, or anal sex. About 15% of male partners develop an itchy rash on the penis after unprotected sex with someone who has an active yeast infection, with uncircumcised men and men with diabetes at higher risk.
Female partners can also acquire the fungus during sex. However, routine treatment of an asymptomatic partner isn’t standard practice. If your partner develops symptoms, they should be evaluated and treated separately.
Other Risk Factors
A weakened immune system raises yeast infection risk substantially. Conditions like HIV, or medications that suppress immune function (such as corticosteroids or drugs used after organ transplants), reduce the body’s ability to keep Candida populations small.
Stress and sleep deprivation don’t directly cause yeast infections, but they impair immune function in ways that can lower your resistance. Similarly, staying in a hot tub for long periods or wearing tight, non-breathable pants regularly can create the warm, humid microenvironment Candida prefers.
Recognizing the Symptoms
The hallmark symptoms are intense itching and irritation of the vulva and vaginal opening, along with a thick, white, clumpy discharge often compared to cottage cheese. You may also notice burning during urination or sex, redness, and swelling of the vulvar tissue. The discharge typically doesn’t have a strong odor. If you notice a fishy smell, that points more toward bacterial vaginosis, a different condition caused by bacterial imbalance rather than fungal overgrowth.
Over-the-counter antifungal treatments work well for occasional, straightforward yeast infections. But if you’re experiencing symptoms for the first time, if they don’t resolve with treatment, or if infections keep coming back, getting a proper diagnosis matters. Several other conditions mimic yeast infection symptoms, and treating the wrong one delays relief.
When Yeast Infections Keep Coming Back
Recurrent yeast infections, defined as three or more episodes in a single year, affect fewer than 5% of women. At that frequency, the issue usually isn’t just one isolated trigger but a combination: perhaps a genetic tendency toward a less robust immune response to Candida, combined with ongoing antibiotic use or uncontrolled blood sugar. Some women harbor a Candida species other than the most common one, and these less typical strains can be resistant to standard over-the-counter treatments.
For recurrent infections, longer courses of antifungal medication or maintenance therapy over several months are typically needed. Identifying and addressing underlying contributors, whether that’s switching antibiotics, improving blood sugar control, or changing contraceptive methods, is just as important as treating each individual episode.