Vaginal yeast infections happen when a fungus called Candida, which naturally lives in small amounts in the vagina, grows out of control. Up to 75% of women will experience at least one yeast infection during their reproductive years, making it one of the most common vaginal infections. The overgrowth isn’t usually caused by one single thing but by a shift in the vaginal environment that allows yeast to multiply faster than the body can keep it in check.
The Balance That Keeps Yeast in Check
A healthy vagina is home to a mix of bacteria and a small amount of yeast. The dominant residents are beneficial bacteria called Lactobacillus, which maintain an acidic environment and actively fight yeast overgrowth. They do this in several ways: competing with yeast for space on the vaginal walls, producing lactic acid that keeps pH low, and releasing natural antifungal compounds including hydrogen peroxide.
When something disrupts this bacterial community, yeast gets an opening. The Candida cells shift from a harmless round form into a more aggressive, thread-like form that can penetrate tissue and cause the itching, burning, and discharge that define a yeast infection. The key point is that you don’t “catch” yeast from somewhere. It’s already there. The infection starts when the conditions inside the vagina change enough to let it take over.
Antibiotics Are the Most Common Trigger
Taking antibiotics, especially broad-spectrum ones prescribed for infections elsewhere in your body, is one of the most well-established causes of yeast infections. Antibiotics can’t distinguish between harmful bacteria and the protective Lactobacillus in your vagina. When those beneficial bacteria are wiped out, the acidic environment weakens and yeast multiplies rapidly to fill the gap. This is why yeast infections commonly show up during or shortly after a course of antibiotics for something entirely unrelated, like a sinus infection or urinary tract infection.
Hormones Play a Major Role
Higher estrogen levels change the vaginal lining in ways that favor yeast growth. Estrogen increases the amount of glycogen (a type of sugar) stored in vaginal tissue, and Candida feeds on it. This is why yeast infections are more common during pregnancy, when estrogen levels surge. Hormonal birth control pills and hormone replacement therapy can have the same effect by raising estrogen levels enough to shift the vaginal environment.
The hormonal connection also explains why yeast infections are far more common during the reproductive years and less common before puberty and after menopause, when estrogen levels are naturally lower. Some women notice a pattern of infections tied to their menstrual cycle, typically in the days before their period when hormonal shifts are most dramatic.
Health Conditions That Raise Your Risk
Diabetes is a significant risk factor, particularly when blood sugar is poorly controlled. Elevated blood sugar translates to higher sugar levels in vaginal secretions, giving yeast more fuel to grow. Keeping blood sugar well managed reduces this risk considerably.
A weakened immune system, whether from HIV, cancer treatment, or medications like corticosteroids, also increases susceptibility. Your immune system normally helps keep Candida populations small even when vaginal bacteria fluctuate. When that immune surveillance drops, yeast infections become more frequent and harder to treat. Roughly 138 million women worldwide deal with recurrent yeast infections each year, defined as four or more episodes in 12 months, and immune suppression is a common factor in those cases.
Moisture and Clothing Choices
Yeast thrives in warm, moist environments. Wearing synthetic underwear or tight-fitting pants traps heat and moisture against the skin, creating conditions that encourage fungal growth. Cotton underwear wicks away excess sweat and moisture, which is why it’s consistently recommended over nylon or polyester. Even underwear with a small cotton panel in the crotch doesn’t fully compensate for the synthetic fabric surrounding it.
Sitting in a wet swimsuit or sweaty workout clothes for extended periods has the same effect. Loose-fitting bottoms and changing out of damp clothing promptly improve airflow to the area and reduce the moisture yeast depends on. Panty liners, though they seem like they’d help, actually decrease breathability and can contribute to irritation.
Douching and Scented Products
Douching disrupts the natural balance of bacteria and acidity that protects the vagina. It flushes out the beneficial Lactobacillus along with everything else, creating an opportunity for yeast to overgrow. The vagina is self-cleaning, and douching introduces an unnecessary disruption that consistently raises infection risk.
Scented tampons, pads, sprays, and powders pose a similar problem. The chemicals in fragrances can irritate vaginal tissue and alter the microbial environment. Switching to unscented products is one of the simplest ways to lower your risk.
Sexual Activity and Yeast Infections
Yeast infections are not sexually transmitted infections. You can get one without ever having sex. That said, they are more common in people who are sexually active. Intercourse can introduce friction and minor tissue irritation that makes the vaginal lining more vulnerable. Semen is alkaline, which temporarily raises vaginal pH and can briefly weaken the acidic defenses that keep yeast in check.
If you already have a yeast infection, sex can make symptoms worse and cause additional pain. A partner can occasionally develop temporary irritation from contact with Candida, but passing a yeast infection back and forth the way you would an STI is not how these infections typically work.
Why Some Women Get Infections Repeatedly
Not all Lactobacillus species protect equally against yeast. Women whose vaginal microbiome is dominated by the most protective species, L. crispatus, tend to be more resistant to infection. Those with higher levels of a less helpful species, L. iners, may actually be more vulnerable. Research has shown that L. iners can enhance the ability of Candida to form stronger, more resilient colonies, essentially making infections worse rather than preventing them. This difference in bacterial composition is largely outside your control and helps explain why some women deal with recurrent infections despite doing everything “right.”
Genetics, individual immune responses, and the specific strains of Candida present in your body all contribute to personal susceptibility. If you’re getting four or more infections per year, the underlying cause is likely a combination of these biological factors rather than a hygiene issue.