Vaginal yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, grows out of control. About 70 to 75% of women will experience at least one yeast infection during their lifetime, making it the second most common vaginal infection in the United States. The overgrowth isn’t usually caused by picking up the fungus from somewhere new. It’s almost always a shift in your body’s internal environment that lets a resident organism flourish.
What Keeps Yeast in Check
A healthy vagina hosts a community of beneficial bacteria, primarily various species of Lactobacillus. These bacteria produce lactic acid, hydrogen peroxide, and other antimicrobial compounds that maintain a vaginal pH between 4 and 4.5. At that acidity level, Candida stays in its harmless, round yeast form and can’t easily multiply or attach to vaginal tissue.
When conditions change and this bacterial balance is disrupted, Candida shifts into an aggressive form, growing long filaments called hyphae. These filaments anchor to the vaginal lining, begin invading tissue, and release a toxin that damages cells. Your immune system responds by flooding the area with inflammatory cells, but they often can’t clear the fungus effectively. Instead, they create a cycle of inflammation and tissue irritation that produces the hallmark symptoms: itching, burning, swelling, and thick, white discharge that typically has no strong odor.
Antibiotics Are a Major Trigger
Broad-spectrum antibiotics are one of the most common causes of yeast infections because they kill off protective Lactobacillus bacteria along with whatever infection they’re treating. Without those bacteria maintaining acidity and producing antifungal compounds, Candida faces little resistance.
A large surveillance study tracking over 31,000 women on antibiotics found the risk of developing a vaginal yeast infection peaked during the second week of treatment, when it was nearly 11 times higher than the baseline risk. The elevated risk persisted through the first and third weeks as well. This pattern held across five different commonly prescribed antibiotics. If you’ve ever noticed a yeast infection appearing partway through or shortly after a course of antibiotics, this is why.
How Hormones Fuel Yeast Growth
Estrogen plays a direct role in creating conditions that favor Candida. Higher estrogen levels stimulate the vaginal lining to produce more glycogen, a sugar-based molecule that essentially feeds the fungus. But estrogen does something else that’s less obvious: it helps Candida evade the immune system. When exposed to estrogen, yeast cells coat themselves with a human protein called Factor H, which acts like a disguise. Immune cells that would normally engulf and destroy the fungus become significantly less effective at recognizing it.
This is why yeast infections are especially common during pregnancy, when estrogen levels surge. It also explains why high-estrogen oral contraceptives and hormone replacement therapy increase risk. Many women notice infections clustering around certain points in their menstrual cycle, when estrogen naturally fluctuates.
Blood Sugar and Diabetes
Poorly controlled blood sugar creates a similar feeding opportunity for Candida. When glucose levels run high, glycogen accumulates in vaginal tissue, lowering pH in ways that paradoxically favor fungal colonization rather than suppress it. The sugar-rich environment also helps Candida build protective biofilms, structured colonies that are harder for both your immune system and antifungal treatments to penetrate.
Research has found that women with recurrent yeast infections tend to have higher average blood sugar levels (as measured by HbA1c) than women without them. For women with diabetes, keeping blood sugar well managed is one of the most effective ways to reduce yeast infection frequency.
Clothing, Moisture, and Local Environment
The vulvovaginal area is already warm and somewhat moist by nature. Anything that traps additional heat and moisture tips the balance further toward fungal growth. Synthetic underwear fabrics like nylon absorb less sweat than cotton, creating a humid microenvironment where Candida thrives. Tight-fitting clothing compounds the problem by increasing friction and reducing airflow.
Research on underwear textiles confirms that moisture retention, heat accumulation, and friction from synthetic or tight fabrics can disrupt the vaginal microbiome and impair the mucosal defenses that normally keep yeast in check. Choosing breathable, cotton-based underwear and avoiding prolonged time in wet swimsuits or sweaty workout clothes are simple ways to reduce this particular risk factor.
Sexual Activity and Transmission
Yeast infections are not classified as sexually transmitted infections because they frequently occur without any sexual contact. The overgrowth originates from Candida already present in your body. That said, sexual activity can play a role. It’s possible to get a yeast infection from a partner, and about 15% of male partners develop an itchy rash on the penis after unprotected sex with someone who has an active infection. Oral sex, new sexual partners, and increased frequency of intercourse can all shift the vaginal environment enough to trigger overgrowth in some women.
Other Common Triggers
Several additional factors can disrupt the vaginal ecosystem and set the stage for an infection:
- Douching and scented products. Vaginal douches, fragranced soaps, and scented tampons or pads can strip away protective bacteria and alter pH.
- Weakened immune system. Conditions or medications that suppress immune function, including corticosteroids and chemotherapy, make it harder for your body to keep Candida under control.
- Stress and sleep deprivation. Both can dampen immune responses enough to allow opportunistic organisms to gain a foothold.
- Staying in wet clothing. Damp environments from swimsuits or exercise gear provide ideal conditions for fungal proliferation.
What a Yeast Infection Looks and Feels Like
The most recognizable symptom is intense itching in and around the vagina. Most women also notice a thick, white discharge often compared to cottage cheese. Unlike bacterial vaginosis or other vaginal infections, yeast infections typically produce little to no odor. Burning during urination or intercourse, redness, and swelling of the vulva are also common.
Vaginal pH during a yeast infection usually stays in the normal range (4.5 or below), which is one way clinicians distinguish it from bacterial vaginosis and other conditions that raise pH. A definitive diagnosis involves examining a sample of discharge under a microscope for fungal structures, though microscopy only catches about 50% of cases, so a culture may be needed if symptoms are present but the initial test is negative.
The Role of Probiotics in Prevention
Because Lactobacillus bacteria are the vagina’s primary defense against Candida, there’s growing interest in whether probiotic supplements can help prevent infections. Two well-studied strains, Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14, have shown strong antifungal activity in both lab and clinical settings. In one randomized trial, these strains significantly reduced vaginal yeast colonization. Lab studies found that their metabolic byproducts alone inhibited fungal growth by roughly 70%, and they were able to completely shut down the metabolic activity of Candida cells.
These probiotics appear to work through multiple mechanisms: producing acids and antimicrobial compounds, physically clustering around yeast cells to prevent them from colonizing tissue, and competing for the same resources. While probiotics aren’t a guaranteed fix, they represent a promising option for women dealing with recurrent infections, particularly after antibiotic use disrupts the natural bacterial community.