Yeast infections are caused by an overgrowth of a fungus called Candida that normally lives in and on your body in small amounts. Up to 75% of women will experience at least one vaginal yeast infection in their lifetime. The fungus isn’t something you “catch” in most cases. It’s already there, kept in check by your body’s natural defenses, until something tips the balance.
How Candida Goes From Harmless to Problematic
Your body can tolerate a low, moderate, or even high number of Candida cells without any symptoms. The fungus sits quietly on skin and mucosal surfaces, coexisting with the rest of your microbiome. Problems start when something disrupts the balance that keeps it in check, allowing the yeast to shift into an aggressive growth phase.
When conditions favor overgrowth, Candida changes shape. It transforms from round, budding yeast cells into elongated, thread-like filaments that can penetrate tissue. This shape change is the key step that turns a harmless resident into something that causes itching, burning, and discharge. Body temperature, contact with surfaces, and certain chemical signals all encourage this transformation, but the biggest factor is usually a disruption to the environment that was keeping the fungus contained.
The Role of Protective Bacteria
In a healthy vaginal environment, Lactobacillus bacteria dominate. These bacteria produce lactic acid, hydrogen peroxide, and other compounds that maintain an acidic pH (typically around 3.8 to 4.2) and directly suppress Candida growth. One species, Lactobacillus crispatus, can actually block Candida from forming those invasive filaments by turning down the genes the fungus needs to become aggressive.
Anything that reduces Lactobacillus populations opens the door for yeast. Antibiotics are one of the most common culprits. A course of antibiotics taken for a sinus infection or UTI doesn’t just target the bacteria making you sick. It also kills off protective vaginal bacteria, leaving a gap that Candida is well positioned to fill. This is why yeast infections so often follow antibiotic use.
Estrogen and Hormonal Changes
Estrogen is one of the strongest drivers of yeast overgrowth, which explains why yeast infections cluster around specific life stages and hormonal events. High estrogen levels directly stimulate Candida to form those tissue-penetrating filaments. Estrogen also suppresses parts of the local immune response that would otherwise keep the fungus contained.
This connection shows up clearly in several situations:
- Pregnancy: Estrogen levels rise dramatically, making yeast infections significantly more common, especially in the second and third trimesters.
- Hormonal contraceptives: Birth control pills that contain estrogen can increase your risk, particularly higher-dose formulations.
- Hormone replacement therapy: Postmenopausal women taking estrogen-based therapy face a higher likelihood of vaginal yeast overgrowth.
- Menstrual cycle fluctuations: Some women notice infections tend to appear at predictable points in their cycle when estrogen peaks.
Blood Sugar and Diabetes
Candida thrives on sugar. When blood glucose levels run high, more glucose ends up in vaginal secretions, essentially feeding the yeast. Women with poorly controlled diabetes are at notably higher risk for acute, chronic, and recurrent vaginal yeast infections. The risk drops when blood sugar is well managed, which is one reason doctors sometimes check blood glucose in women who keep getting yeast infections without an obvious cause.
Habits That Shift the Balance
Several everyday habits can create conditions that favor yeast overgrowth, mostly by disrupting pH, killing protective bacteria, or trapping moisture.
Douching is one of the biggest offenders. It strips away healthy bacteria and disrupts the natural acid balance that keeps Candida in check. The vagina is self-cleaning, and introducing water, vinegar, or commercial douching products does more harm than good. Scented soaps, fragranced pads and tampons, and “feminine hygiene” washes can cause similar disruption. Unscented products for external use only are a safer choice.
Clothing also plays a role. Tight, non-breathable fabrics trap heat and moisture against the skin, creating exactly the warm, damp environment where yeast grows most easily. Wearing synthetic underwear or staying in sweaty workout clothes for extended periods raises your risk. Cotton underwear and changing out of damp clothing promptly are simple ways to reduce it.
Sexual Activity and Transmission
A yeast infection is not classified as a sexually transmitted infection because you can develop one without any sexual contact. That said, it is possible to pass yeast to a partner 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. Sexual intercourse can also introduce friction and alter vaginal pH, which may trigger an infection in someone already prone to them.
Why Some People Get Recurring Infections
Recurrent vulvovaginal candidiasis, defined as three or more symptomatic episodes in a single year, affects fewer than 5% of women. For these individuals, the causes are often layered: a combination of genetic immune factors, ongoing hormonal influences, and persistent microbiome disruption that keeps tipping the balance back toward overgrowth.
Some recurrent infections involve Candida species other than the most common one (Candida albicans). These less common species, such as Candida glabrata, can be more resistant to standard over-the-counter treatments. If you’ve treated a yeast infection with a typical antifungal and symptoms keep returning or never fully resolve, a different species may be involved, and testing can help identify the right approach.
Immune Suppression and Other Medical Factors
Anything that weakens your immune system makes it harder for your body to keep Candida in check. This includes conditions like HIV, medications that suppress immune function (such as corticosteroids or drugs used after organ transplants), and the general immune dip that comes with severe illness or major surgery. Chemotherapy, which can damage mucosal barriers throughout the body, also increases vulnerability to yeast overgrowth in multiple areas, not just the vagina.
Stress and sleep deprivation don’t cause yeast infections on their own, but chronic stress can dampen immune function enough to make infections more likely in someone who already has other risk factors in play.