Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, grows out of control. Up to 75% of women will experience at least one vaginal yeast infection in their lifetime, and the triggers range from antibiotics to tight clothing to hormonal shifts. Understanding what sets off that overgrowth can help you avoid repeat infections.
What Actually Happens Inside Your Body
Candida lives in the vagina of many healthy women without causing any problems. In its harmless form, it exists as a round yeast cell. The trouble starts when it shifts into a more aggressive form, sprouting long filaments called hyphae that can physically penetrate the vaginal lining. When enough of these filaments accumulate, your immune system launches an inflammatory response, and that’s when you feel the burning, itching, and swelling of an active infection.
What keeps Candida in check? Mostly a group of beneficial bacteria called Lactobacillus. These bacteria produce lactic acid that keeps vaginal pH below 4.5, an environment acidic enough to prevent Candida from shifting into its aggressive form. They also compete directly with Candida for space on the vaginal walls and release natural antifungal compounds. When something disrupts these protective bacteria or weakens your immune defenses, Candida seizes the opportunity to multiply.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics don’t just kill the bacteria making you sick. They also wipe out the Lactobacillus that protect your vagina. With those beneficial bacteria reduced, there’s less lactic acid keeping the pH low and fewer organisms competing with Candida for resources. The fungus can then multiply rapidly and shift into its invasive form. This is why yeast infections so often follow a course of antibiotics for a sinus infection, urinary tract infection, or strep throat.
Not every antibiotic course will trigger a yeast infection, but the broader the antibiotic (meaning it targets many types of bacteria), the higher the risk. If you’ve noticed a pattern of yeast infections after taking antibiotics, that’s worth mentioning to your doctor, who may suggest preventive steps for future courses.
Hormonal Changes and Estrogen
Estrogen plays a direct role in the vaginal environment. It increases glycogen (a type of sugar) in vaginal tissue, which feeds both Lactobacillus and Candida. Under normal conditions, Lactobacillus wins that competition. But when estrogen levels spike, the extra glycogen can tip the balance toward Candida overgrowth.
This is why yeast infections are more common during pregnancy, when estrogen levels are significantly elevated. Many women also notice infections flaring in the days before their period, when hormonal shifts alter the vaginal environment. Hormonal birth control, particularly higher-estrogen formulations, can have a similar effect. Women going through menopause sometimes experience the opposite problem: low estrogen reduces the Lactobacillus population, which can also leave the door open for Candida.
High Blood Sugar Feeds the Fungus
Diabetes is a well-established risk factor. When blood sugar runs high, excess sugar can show up in vaginal secretions and urine, essentially providing extra fuel for yeast to grow. The CDC notes that women with diabetes are more prone to yeast infections for exactly this reason. Poorly controlled blood sugar creates a near-constant environment that favors fungal overgrowth, which is why recurrent yeast infections sometimes prompt doctors to screen for diabetes.
Even without diabetes, diets very high in sugar may contribute to the problem for some women, though the evidence here is less definitive than for clinical hyperglycemia.
Douching and Scented Products
The vagina is self-cleaning, and douching disrupts that system. It washes away beneficial bacteria and alters the natural acidity that keeps infections at bay. The Office on Women’s Health is direct about this: douching can cause an overgrowth of harmful organisms, leading to yeast infections or bacterial vaginosis.
Scented tampons, pads, powders, and sprays pose a similar risk. These products can irritate vaginal tissue and shift the microbial balance. Unscented products and plain water for external washing are all you need.
Moisture and Tight Clothing
Candida thrives in warm, dark, moist environments. Sitting in a wet bathing suit or sweaty workout clothes for hours creates ideal conditions for overgrowth. The fix is straightforward: change out of wet or damp clothing as soon as possible, choose moisture-wicking fabrics for exercise, and opt for cotton underwear and looser-fitting bottoms, especially in warmer months. Tight synthetic underwear traps heat and moisture against the skin, which is exactly what the fungus needs to proliferate.
A Weakened Immune System
Your immune system normally keeps Candida populations small even without you noticing. Conditions that suppress immune function, such as HIV, chemotherapy, or long-term corticosteroid use, remove that surveillance. Without a robust immune response, Candida can grow unchecked and shift into its invasive form more easily. Chronic stress and sleep deprivation also impair immune function, though their contribution to yeast infections is harder to measure precisely.
Can You Get It From Sex?
A yeast infection is not a sexually transmitted infection. You can develop one without any sexual contact at all. That said, sex can play a role. About 15% of male partners develop an itchy rash on the penis after unprotected sex with someone who has an active yeast infection. Oral sex and the introduction of saliva can also alter vaginal pH. Semen is less acidic than the vagina, so frequent unprotected sex can temporarily shift the environment in a direction that favors yeast growth.
How to Tell It’s a Yeast Infection
The hallmark symptoms are intense itching, redness and swelling of the vulva, burning during urination or sex, and a thick, white, cottage cheese-like discharge that typically has little to no odor. This discharge is a key distinguishing feature. Bacterial vaginosis, which is often confused with yeast infections, produces a thin, grayish discharge that tends to have a noticeable fishy smell. The two conditions require different treatments, so getting the right diagnosis matters, especially if it’s your first time experiencing symptoms or if over-the-counter antifungal treatments aren’t working.
Reducing Your Risk
Most prevention strategies come down to protecting the Lactobacillus population that keeps Candida in check. Avoiding unnecessary antibiotics, skipping douches and scented products, managing blood sugar if you have diabetes, and changing out of wet clothes promptly all help maintain that balance.
Probiotics show genuine promise. Clinical trials have found that specific Lactobacillus strains, particularly L. crispatus, can reduce both the amount of discharge and the itching associated with yeast infections. In one study, a combination of probiotic strains taken over six months led to significantly fewer recurrences compared to a placebo group. Not all probiotic supplements contain effective strains, though, so look for products that specify L. crispatus, L. acidophilus, or L. rhamnosus. Interestingly, not all vaginal Lactobacillus species are equally protective. One species, L. iners, has actually been shown to enhance Candida’s ability to form resilient colonies, which may partly explain why some women with seemingly normal bacterial populations still get infections.
Cotton underwear, loose clothing, and wiping front to back are small habits that reduce moisture and limit the spread of organisms from the digestive tract to the vaginal area. None of these steps guarantee prevention, but together they create an environment where Candida is far less likely to gain the upper hand.