Yeast infections happen when a fungus called Candida, which normally lives in small amounts in and on your body, grows out of control. About 75% of women will experience at least one vaginal yeast infection in their lifetime, and roughly 138 million women worldwide deal with recurrent infections (four or more per year). The overgrowth isn’t random. It’s triggered by specific shifts in your body’s chemistry, immune defenses, or environment that tip the balance in the fungus’s favor.
The Balance That Keeps Yeast in Check
Your vagina naturally hosts a community of microorganisms, and under normal conditions, beneficial bacteria called Lactobacillus dominate that community. These bacteria produce lactic acid, which keeps the vaginal environment acidic, typically between a pH of 3.8 and 4.5. That acidity is inhospitable to Candida. Lactobacillus also secretes other metabolites that directly suppress fungal growth, essentially acting as a natural defense system.
A yeast infection starts when something disrupts this balance. When Lactobacillus populations drop, the pH rises, the chemical signals keeping Candida in check weaken, and the fungus transitions from a harmless passenger to an active infection. It shifts from a round, inactive yeast form to an elongated form that can penetrate the vaginal lining and trigger inflammation, itching, and discharge.
Antibiotics Are the Most Common Trigger
Antibiotics are designed to kill bacteria, but they don’t discriminate between harmful bacteria and the protective Lactobacillus in your vagina. A course of antibiotics for a sinus infection, urinary tract infection, or anything else can wipe out enough of that beneficial bacteria to open the door for Candida. This is the single most common reason otherwise healthy women develop yeast infections. The broader the antibiotic (meaning it targets a wider range of bacteria), the higher the risk.
How Hormones Fuel Yeast Growth
Estrogen plays a direct role in creating an environment where Candida thrives. Higher estrogen levels cause the cells lining the vagina to accumulate more glycogen, a stored form of sugar. When those cells shed naturally, the glycogen breaks down into simpler sugars that feed both Lactobacillus and Candida. Under normal circumstances, Lactobacillus wins that competition. But when estrogen levels spike, the extra glycogen can tip the balance toward yeast overgrowth.
This is why yeast infections are more common during pregnancy, when estrogen levels are significantly elevated. It also explains why hormonal contraceptives, particularly higher-dose birth control pills, increase risk. Women in the luteal phase of their menstrual cycle (the two weeks before a period) sometimes notice recurring infections for the same reason: estrogen and progesterone fluctuations change the vaginal environment just enough to let Candida gain a foothold.
Blood Sugar and Diabetes
Elevated blood sugar creates a two-pronged problem. First, it gives Candida more fuel. The fungus feeds on glucose, and when your blood sugar runs high, glucose levels rise throughout your body, including in vaginal secretions. Second, high glucose actively weakens your immune response. It impairs the ability of immune cells to find, engulf, and kill Candida. Research shows that at higher glucose concentrations, Candida becomes better at adhering to tissue and resisting the body’s defenses.
Women with uncontrolled or poorly managed diabetes are significantly more likely to develop yeast infections, and those infections tend to be more stubborn. Recurrent yeast infections can sometimes be an early sign that blood sugar levels aren’t well controlled, even before a diabetes diagnosis.
Immune System Suppression
Your immune system normally keeps Candida populations small even when other conditions shift. When that immune surveillance weakens, yeast can proliferate unchecked. Several situations suppress the relevant immune responses:
- HIV/AIDS directly impairs the immune cells responsible for controlling fungal infections
- Steroids (like prednisone taken for asthma, autoimmune conditions, or inflammation) dampen the immune response broadly
- Chemotherapy suppresses immune function as a side effect of targeting rapidly dividing cells
- Chronic stress and sleep deprivation reduce immune efficiency over time, though less dramatically than the factors above
For people with severely weakened immune systems, yeast infections can become chronic and harder to treat, sometimes requiring longer or more aggressive approaches.
Moisture, Heat, and Clothing Choices
Candida thrives in warm, moist environments. Anything that traps heat and moisture against the skin creates conditions that favor yeast growth. The CDC specifically recommends wearing cotton underwear and breathable, non-restrictive clothing to reduce risk. Synthetic fabrics like nylon and polyester don’t wick moisture the way cotton does, so sweat and vaginal moisture sit against the skin longer.
Sitting in a wet swimsuit, exercising in tight leggings without changing afterward, or wearing pantyhose for extended periods all create that warm, damp microclimate. The fix is straightforward: change out of wet or sweaty clothing promptly and opt for breathable fabrics when possible. These habits won’t prevent every infection, but they remove one contributing factor.
Other Contributing Factors
Douching and scented vaginal products disrupt the natural bacterial balance in a way similar to antibiotics. They wash away or chemically alter the Lactobacillus population, raising pH and giving Candida room to grow. Plain water for external cleaning is sufficient; the vagina is self-cleaning internally.
Sexual activity doesn’t cause yeast infections (they aren’t sexually transmitted), but it can sometimes contribute. Friction, lubricants, or spermicides may irritate the vaginal lining, and oral sex can introduce bacteria that shift the microbial balance. New sexual partners correlate with changes in vaginal flora, though the relationship is indirect.
Diet alone rarely causes a yeast infection in someone with normal blood sugar, but a diet extremely high in refined sugar and simple carbohydrates may modestly contribute by raising blood glucose levels. The effect is far less significant than the factors above.
Why Some People Get Recurring Infections
About 5 to 8% of women experience recurrent yeast infections, defined as four or more episodes per year. Globally, that translates to roughly 138 million women annually. Recurrence often signals an underlying factor that hasn’t been addressed: uncontrolled blood sugar, ongoing antibiotic use, hormonal changes, or immune suppression.
In some cases, the culprit is a less common species of Candida. Most yeast infections are caused by Candida albicans, which responds well to standard antifungal treatments. But species like Candida glabrata are naturally more resistant to common antifungals. Around 5% of invasive C. glabrata infections resist at least one antifungal medication. If you’re treating infections with over-the-counter remedies and they keep coming back, a lab test of vaginal fluid can identify the exact species and guide more effective treatment.
Recurrent infections can also result from incomplete treatment. Symptoms often improve within a few days of starting antifungal medication, but the fungus may not be fully eliminated. Stopping treatment early allows surviving yeast to repopulate quickly.
How a Yeast Infection Is Diagnosed
Symptoms like itching, thick white discharge, and burning overlap with other conditions, including bacterial vaginosis and some sexually transmitted infections. A clinical diagnosis involves a pelvic exam to look for visible signs of infection, followed by testing a sample of vaginal fluid. The lab can confirm whether yeast is present and identify the species. This matters because treatment that works for Candida albicans may not work for other species, and misdiagnosis leads to ineffective treatment and frustration.
If you’ve had yeast infections before and recognize the symptoms clearly, over-the-counter antifungal treatments are reasonable for an occasional, straightforward episode. But if infections recur, symptoms are unusual, or treatment isn’t working, lab testing becomes important to make sure the diagnosis is correct and the right approach is being used.