Yeast infections happen when a fungus called Candida, which normally lives in small amounts on your skin and inside your body, grows out of control. The most common location is the vagina, where roughly 75% of women will experience at least one yeast infection in their lifetime. The overgrowth isn’t caused by “catching” something new. It’s triggered when something disrupts the environment that normally keeps Candida in check.
The Balance That Keeps Yeast Under Control
Your vagina naturally contains both Candida yeast and beneficial bacteria, primarily Lactobacillus species. These bacteria metabolize glycogen (a sugar stored in vaginal tissue) into lactic acid, maintaining an acidic environment with a pH around 3.8 to 4.5. That acidity suppresses Candida growth and keeps the yeast population small and harmless.
Not all Lactobacillus species offer the same protection. Research published in the American Journal of Obstetrics and Gynecology found that women whose vaginal microbiome was dominated by Lactobacillus crispatus had significantly lower rates of Candida colonization compared to those dominated by Lactobacillus iners. In lab experiments, L. crispatus produced greater concentrations of lactic acid and showed stronger pH-dependent growth inhibition of Candida. Women with L. iners-dominant communities were nearly three times more likely to harbor Candida. This helps explain why some women seem more prone to yeast infections than others, even without an obvious trigger.
Anything that weakens those protective bacteria, raises vaginal pH, or feeds the yeast can tip the balance toward overgrowth.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics are one of the most frequent causes of yeast infections. They kill bacteria indiscriminately, wiping out the protective Lactobacillus alongside the harmful bacteria they’re prescribed to treat. With that bacterial defense reduced, Candida faces less competition and can multiply rapidly. This is why many women develop a yeast infection during or shortly after a course of antibiotics for a completely unrelated problem, like a sinus infection or urinary tract infection.
How Hormones Change Your Risk
Estrogen plays a direct role in vaginal ecology. It stimulates glycogen production in vaginal tissue, which feeds the Lactobacillus that maintain acidity. But elevated estrogen also increases the amount of sugar available for Candida to consume, and at high enough levels, it can shift the balance in favor of yeast overgrowth.
This is why yeast infections are more common during pregnancy, when estrogen levels surge. Women using hormonal contraceptives, particularly high-dose estrogen formulations, and those on hormone replacement therapy also face increased risk. On the other end of the spectrum, postmenopausal women who are not on hormone therapy experience estrogen deficiency that thins the vaginal lining and raises pH to 6 or higher, which creates a different set of vulnerabilities to infection.
Many women notice yeast infections tend to pop up at certain points in their menstrual cycle, typically in the days before a period, when hormonal shifts are most pronounced.
High Blood Sugar Feeds the Yeast
Uncontrolled diabetes is a well-established risk factor. When blood sugar runs high, excess glucose shows up in mucous membranes and is released in urine, creating a sugar-rich environment where yeast thrives. According to the CDC, this excess sugar directly encourages yeast and bacteria to grow. Women with poorly controlled type 1 or type 2 diabetes often experience recurrent yeast infections, and getting blood sugar under consistent control is one of the most effective ways to break that cycle.
You don’t need a diabetes diagnosis for this to matter. Any period of consistently elevated blood sugar, whether from diet, medication side effects, or undiagnosed insulin resistance, can raise your risk.
A Weakened Immune System
Your immune system normally keeps Candida populations in check. When that immune surveillance weakens, yeast can grow unchecked. People living with HIV/AIDS face significantly higher rates of Candida infections, and esophageal candidiasis (a yeast infection in the throat) is one of the most common infections associated with advanced HIV.
Corticosteroids also suppress immune function. Inhaled corticosteroids used for asthma, for example, are a recognized risk factor for oral yeast infections (thrush). Systemic steroids prescribed for autoimmune conditions carry similar risks. Chemotherapy and other immunosuppressive medications can also open the door to Candida overgrowth throughout the body.
Hygiene Products That Disrupt Vaginal Chemistry
The vagina is self-cleaning, and many products marketed for “intimate hygiene” actually increase infection risk by disrupting its natural microbial balance. Douching has long been linked to negative impacts on vaginal health, but newer research shows other products are problematic too.
Vaginal gel sanitizers are associated with an eight-fold higher risk of yeast infection and nearly a 20-fold higher risk of bacterial infection. Lubricants and moisturizing creams correlated with a 2.5 times higher risk of yeast infection. The likely mechanism is that certain ingredients in these products kill off the protective bacteria that would otherwise keep Candida in check. Scented soaps, bubble baths, and fragranced period products can cause similar disruption when they come into contact with vaginal tissue.
Clothing and Moisture
Candida thrives in warm, moist environments. Tight, non-breathable clothing and synthetic underwear trap heat and moisture against the skin, creating ideal conditions for yeast to multiply. The Cleveland Clinic recommends 100% cotton underwear because it wicks away excess sweat and moisture that yeast feeds on. Synthetic fabrics with a small cotton crotch panel don’t offer the same protection, since the surrounding synthetic material still limits airflow.
Sitting in a wet swimsuit or sweaty workout clothes for extended periods creates the same problem. Changing out of damp clothing promptly reduces the time yeast has to proliferate in that warm, moist environment.
Sex Can Play a Role, But It’s Not an STI
A yeast infection is not a sexually transmitted infection. You can develop one without any sexual contact. That said, sex can contribute to yeast overgrowth in a few ways. Friction and the introduction of saliva, lubricants, or a partner’s natural flora can alter vaginal pH and microbial balance. About 15% of men develop an itchy rash on the penis after unprotected sex with a woman who has an active yeast infection, so passing Candida back and forth between partners is possible, though it’s not the primary way infections develop.
When Infections Keep Coming Back
Recurrent vulvovaginal candidiasis is defined as three or more symptomatic episodes within a single year. It affects fewer than 5% of women but carries a significant quality-of-life and economic burden. If your yeast infections keep returning, it usually signals an ongoing underlying factor: a vaginal microbiome that’s naturally less protective, a hormonal pattern that favors overgrowth, uncontrolled blood sugar, or repeated exposure to a trigger like antibiotics or irritating products.
Identifying and addressing that underlying driver is more effective than treating each individual episode. For some women, this means switching birth control methods, improving blood sugar management, eliminating problematic hygiene products, or working with a provider on a longer-term antifungal strategy.