The main cause of a yeast infection is an overgrowth of *Candida*, a fungus that naturally lives in the vagina in small amounts. Under normal conditions, beneficial bacteria keep *Candida* in check. When something disrupts that balance, the fungus multiplies rapidly and causes symptoms like itching, burning, and thick discharge. The disruption can come from antibiotics, hormonal shifts, high blood sugar, or anything that changes the vaginal environment enough to let yeast gain the upper hand.
How Your Body Normally Keeps Yeast in Check
Your vagina is home to a community of microorganisms, and the most important residents are *Lactobacillus* bacteria. These bacteria ferment glycogen (a sugar stored in vaginal tissue) and produce lactic acid, which keeps vaginal pH at roughly 3.8 to 4.5. That acidic environment is hostile to most harmful organisms, including *Candida*. *Lactobacillus* also produces hydrogen peroxide, which adds another layer of defense against infection.
When this bacterial population is healthy and dominant, *Candida* stays at low levels and causes no symptoms. A yeast infection develops when something reduces the *Lactobacillus* population or changes conditions enough for yeast to grow faster than the bacteria can suppress it.
Antibiotics: The Most Common Trigger
Antibiotics are one of the most frequent triggers for yeast infections because they kill bacteria indiscriminately. When you take antibiotics for a sinus infection, urinary tract infection, or any other bacterial illness, the medication also wipes out *Lactobacillus* in the vagina. With those protective bacteria depleted, the vaginal pH can rise, lactic acid production drops, and *Candida* suddenly has room to multiply. This is why many people notice a yeast infection shortly after finishing a course of antibiotics.
Hormones and Estrogen
Estrogen plays a direct role in vaginal health by driving glycogen production in vaginal tissue. Glycogen is the fuel source that *Lactobacillus* bacteria break down into lactic acid. So under normal circumstances, estrogen actually supports the protective bacterial environment. But the relationship is more complicated than it first appears.
When estrogen levels are especially high, such as during pregnancy, certain phases of the menstrual cycle, or while taking hormonal birth control, the vaginal lining produces more glycogen than usual. *Candida* can also feed on glycogen breakdown products, so an abundance of this sugar can fuel yeast growth at the same time it feeds beneficial bacteria. Pregnancy is a well-known risk period for yeast infections for exactly this reason: estrogen surges create a sugar-rich environment that favors both bacteria and yeast, but the balance tips toward yeast more easily.
On the other end, when estrogen drops significantly (as it does during menopause), glycogen decreases, the *Lactobacillus* population shrinks, and vaginal pH rises. This can also create conditions where *Candida* or other organisms take hold, though bacterial vaginosis becomes more common than yeast infections in this scenario.
Blood Sugar and Diabetes
People with diabetes, particularly when blood sugar is poorly controlled, face a higher risk of yeast infections. Elevated blood glucose means more sugar circulates through the body, including in vaginal secretions. The CDC notes that when blood sugar is high, excess sugar can be released in urine, encouraging yeast and bacteria to grow. *Candida* thrives on sugar, so consistently high glucose levels essentially feed the fungus and make infections more frequent and harder to resolve.
This connection isn’t limited to people with a diabetes diagnosis. Anyone experiencing prolonged blood sugar spikes, whether from diet, medications like corticosteroids, or insulin resistance, may notice more frequent yeast infections.
Immune System Factors
Your immune system actively suppresses *Candida* through specific types of immune cells. Two types of helper T cells are especially important: one produces signaling molecules that activate antifungal defenses, and the other recruits immune cells to the site of infection. Research published in *Frontiers in Immunology* found that people with recurrent yeast infections had significantly lower levels of these protective immune signals compared to people experiencing a one-time infection.
Anything that weakens your immune system can lower these defenses. HIV, chemotherapy, long-term corticosteroid use, chronic stress, and sleep deprivation all reduce immune function and increase susceptibility to yeast overgrowth. This is why recurrent yeast infections (four or more per year) sometimes prompt doctors to screen for underlying immune or metabolic conditions.
Clothing, Moisture, and Daily Habits
The warm, moist environment of the vaginal area is already favorable for fungal growth. Certain clothing choices can make it worse. Research in the *Brazilian Journal of Sexually Transmitted Diseases* found that women who wore synthetic underwear had significantly higher rates of vulvovaginal infections compared to those who didn’t. Tight jeans compound the issue by compressing the area, trapping heat, blocking airflow, and increasing friction, all of which can change local temperature, humidity, and pH.
Cotton underwear is widely recommended because the fabric allows better ventilation. Other practical habits that reduce risk include changing out of wet swimsuits or sweaty workout clothes promptly, avoiding scented soaps or douches in the vaginal area (which can disrupt pH and kill beneficial bacteria), and wiping front to back after using the bathroom.
Which Yeast Species Are Responsible
*Candida albicans* is the species behind most vaginal yeast infections. It is the most common cause overall and the one that standard over-the-counter antifungal treatments are designed to target. However, non-albicans species like *C. glabrata*, *C. parapsilosis*, *C. tropicalis*, and *C. krusei* are increasingly recognized as causes of infection, particularly in people who get recurrent infections or who don’t respond to standard treatment. These other species can be harder to treat because some of them are naturally resistant to common antifungal medications.
What Treatment and Recovery Look Like
Most yeast infections clear up within a few days to a week with antifungal treatment, which is available over the counter as creams or suppositories. More severe infections may take longer. You’ll typically notice itching and burning improve within the first day or two, but it’s important to complete the full course of treatment even if symptoms resolve early, because stopping too soon can allow the yeast to rebound.
For recurrent infections, a doctor may prescribe a longer treatment course or a maintenance regimen to keep yeast levels suppressed over time. If you’re getting yeast infections frequently and the usual treatments aren’t working, it’s worth having the specific *Candida* species identified, since non-albicans species often require different medication.