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. Around 70 to 75% of women will experience at least one vaginal yeast infection during their reproductive years. The overgrowth isn’t caused by poor hygiene or catching something from someone else. It’s the result of a shift in the vaginal environment that gives yeast an opening to thrive.
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 from the Lactobacillus family, which produce lactic acid and keep the vaginal pH between 3.8 and 4.5. That acidic environment is inhospitable to most harmful organisms, including Candida.
Not all Lactobacillus species are equally protective. Research published in the American Journal of Obstetrics & Gynecology found that women whose vaginal flora was dominated by Lactobacillus crispatus were significantly less likely to harbor Candida than women dominated by a different species, Lactobacillus iners. The reason: L. crispatus produces substantially more lactic acid, which directly inhibits Candida growth in a pH-dependent way. Women with L. iners-dominant communities were nearly three times more likely to have Candida colonization. This helps explain why some women seem prone to yeast infections while others rarely get them, even when their habits are similar.
When anything disrupts these protective bacteria or changes the vaginal environment, Candida can shift from a harmless passenger to an overgrown colony that causes itching, burning, and the thick, white, cottage cheese-like discharge that’s the hallmark of a yeast infection.
How Antibiotics Set Off Overgrowth
Antibiotics are one of the most common triggers. They’re designed to kill bacteria, but they don’t distinguish between harmful bacteria causing your sinus infection and the beneficial Lactobacillus keeping your vaginal ecosystem stable. When those protective bacteria are wiped out, yeast faces less competition and fewer growth-suppressing acids, so it multiplies quickly.
The extent of fungal overgrowth varies from person to person. Research from the American Society for Microbiology found that the variation is partly tied to an enzyme called beta-lactamase, which some gut bacteria naturally produce. People with lower levels of this enzyme experienced more Candida growth after antibiotic treatment. This individual variation is why one course of antibiotics might trigger a yeast infection in you but not in a friend taking the same medication.
The Role of Estrogen and Hormonal Shifts
Estrogen plays a direct role in creating conditions that favor yeast. Higher estrogen levels cause vaginal cells to store more glycogen, a form of sugar that Candida feeds on. During the ovulatory phase of the menstrual cycle, when estrogen peaks, free glycogen in the vagina can exceed glucose levels by roughly tenfold. That’s a feast for yeast.
This estrogen connection explains why yeast infections are more common during pregnancy (when estrogen surges), while taking hormonal birth control with higher estrogen doses, and during hormone replacement therapy. It also explains the timing many women notice: infections that seem to flare around ovulation or just before a period, tracking with hormonal shifts throughout the cycle. Before puberty and after menopause, when estrogen levels are low, yeast infections are far less frequent.
Blood Sugar and Diabetes
Yeast feeds on sugar, so elevated blood sugar levels create a more hospitable environment for Candida throughout the body, including the vagina. Higher-than-normal blood glucose raises the sugar content of vaginal secretions and can alter vaginal pH, making the environment less acidic and easier for yeast to colonize.
Women with poorly controlled diabetes are at particularly high risk. The CDC classifies diabetes as a factor in “complicated” yeast infections, meaning they tend to be more severe, harder to treat, and more likely to recur. Getting blood sugar under consistent control is one of the most effective ways to reduce the frequency of yeast infections for women with diabetes.
Immune System Suppression
Your immune system normally helps keep Candida populations small even when the vaginal environment shifts slightly. When immune function is compromised, that surveillance weakens. Women living with HIV have higher rates of vaginal Candida colonization, and the likelihood of symptomatic infections increases as immune suppression becomes more severe.
Other forms of immune suppression carry similar risks. Corticosteroid medications, whether taken for asthma, autoimmune conditions, or after an organ transplant, can dampen the immune response enough to allow yeast overgrowth. Chemotherapy and other immunosuppressive therapies have the same effect. In these cases, yeast infections often don’t respond well to the shorter treatment courses that work for most women, and may require longer or more intensive approaches.
Everyday Habits That Create the Right Conditions
Candida thrives in warm, moist environments, so anything that traps heat and moisture against the vulva can tip the balance. Synthetic underwear fabrics don’t breathe well, holding sweat and moisture close to the skin. Cotton underwear wicks away that excess moisture. Even underwear made from synthetic material with a cotton crotch panel doesn’t offer the same protection, because the surrounding synthetic fabric still limits airflow.
Sitting in a wet swimsuit for hours, wearing tight workout clothes long after exercising, and using scented soaps or douches in the vaginal area can all contribute. Douching is particularly disruptive because it directly washes away Lactobacillus and changes vaginal pH, creating exactly the conditions Candida needs. The vagina is self-cleaning, so these products create more problems than they solve.
Why Some Women Get Repeated Infections
Recurrent yeast infections, defined as four or more episodes in a single year, can be secondary to identifiable causes like frequent antibiotic use, uncontrolled diabetes, or immunosuppression. But in many cases, the recurrence is idiopathic, meaning there’s no obvious underlying trigger. The composition of a woman’s vaginal microbiome, her individual immune response to Candida, and her baseline estrogen levels all contribute to susceptibility in ways that vary widely from person to person.
If you’re getting yeast infections repeatedly, it’s worth looking at the factors you can modify: how well blood sugar is controlled, whether antibiotics are being prescribed when they’re truly necessary, what fabrics are sitting against your skin for most of the day, and whether any hormonal medications might be playing a role. For many women, identifying and addressing even one contributing factor can break the cycle.