A vaginal yeast infection happens when a fungus called Candida, which already lives in small amounts inside the vagina, multiplies beyond what the body can keep in check. About 75% of women will experience at least one yeast infection in their lifetime. It’s not something you “catch” from the outside in most cases. It starts from within, when the vaginal environment shifts in a way that lets yeast thrive.
Yeast Already Lives in Your Body
An estimated 10 to 50% of women of reproductive age carry Candida in their vagina without any symptoms at all. Candida albicans is the species responsible roughly 85 to 90% of the time. In small numbers, it coexists peacefully with the bacteria that make up your vaginal microbiome. The infection starts not because yeast was introduced, but because something changed in the environment that allowed it to grow unchecked.
A healthy vagina maintains a pH between 3.8 and 4.5, which is quite acidic. Beneficial bacteria, primarily Lactobacillus species, are responsible for maintaining that acidity. These bacteria produce antimicrobial substances that suppress yeast growth, and they physically compete with Candida for space on the vaginal lining. When these protective bacteria are reduced or the vaginal chemistry shifts, Candida takes advantage. It transitions from a harmless round cell into an invasive form with branching filaments that penetrate vaginal tissue, causing the itching, burning, and thick white discharge that characterize a yeast infection.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics are one of the most frequent causes of yeast infections. These medications are designed to kill harmful bacteria, but they don’t discriminate. They wipe out the protective Lactobacillus bacteria in the vagina along with whatever infection they were prescribed for. With fewer beneficial bacteria holding the line, yeast populations expand rapidly. This is why yeast infections commonly show up during or shortly after a course of antibiotics for something entirely unrelated, like a sinus infection or strep throat.
Hormones Play a Major Role
Estrogen directly influences the vaginal environment. Higher estrogen levels increase the amount of glycogen (a sugar) stored in vaginal cells, and Candida feeds on sugar. This is why yeast infections are more common during pregnancy, when estrogen levels are significantly elevated. It also explains why some women notice infections around certain points in their menstrual cycle, or after starting hormonal birth control that raises estrogen levels.
The pH of the vagina naturally fluctuates with hormones too. Just before your period and after menopause, pH tends to rise above 4.5, which can create conditions more favorable for yeast. Younger girls who haven’t reached puberty have lower estrogen levels and rarely get vaginal yeast infections, though it’s not impossible.
Moisture and Clothing Choices
Yeast thrives in warm, moist environments, and the vulvar area already checks both boxes. Clothing choices can make this worse. Synthetic fabrics like nylon trap moisture against the skin, creating ideal conditions for fungal overgrowth. Tight-fitting pants, leggings, and underwear compound the problem by limiting airflow.
The University of Iowa Health Care recommends white, all-cotton underwear because cotton allows air circulation and draws moisture away from the skin. Sitting in a wet swimsuit or sweaty workout clothes for extended periods has the same effect as wearing synthetics. Keeping the area dry is one of the simplest ways to reduce your risk. If you tend toward chronic dampness, keeping a spare pair of cotton underwear to change into can help.
Scented Products Disrupt the Balance
The vagina is self-cleaning, and introducing chemical products to the area frequently does more harm than good. Scented tampons, pads, soaps, bubble baths, and sprays can all disrupt the bacterial balance and pH levels that keep yeast in check. Ingredients to watch for include fragrance, chlorine, and titanium dioxide. Even products marketed as “natural” that contain essential oils like lavender, mint, or rose can cause the same disruption. Douching is particularly problematic because it flushes out protective bacteria along with everything else.
Sex Can Trigger It, but It’s Not an STI
Yeast infections are not sexually transmitted infections, but sexual activity can create conditions that trigger one. Semen is alkaline, with a pH around 7 to 8, and introducing it into the acidic vaginal environment temporarily raises the pH. That shift can be enough to give yeast a window to proliferate. Condoms, spermicides, lubricants, and other products used during sex can have a similar pH-disrupting effect.
Penetrative sex can also cause minor irritation to the vaginal lining. If a mild, unnoticeable yeast overgrowth is already underway, that friction can inflame the tissue enough to bring on full symptoms. This makes it seem like the infection was caused by sex, when in reality it was already developing. Oral sex can also introduce bacteria or saliva that alter the vaginal environment.
Other Factors That Raise Your Risk
A weakened immune system makes it harder for your body to keep Candida in check. Conditions like uncontrolled diabetes are a well-known risk factor because elevated blood sugar feeds yeast throughout the body, including the vagina. Chronic stress and lack of sleep suppress immune function in subtler ways that can have the same result over time.
Diet plays a more indirect role. While no single food “causes” a yeast infection, consistently high sugar intake raises blood glucose levels, which can promote yeast growth. Staying in damp environments, using hot tubs frequently, or wearing panty liners with nylon mesh daily can also contribute by keeping the vulvar area warm and moist.
Recurrent Infections
Most women who get a yeast infection experience it as an occasional inconvenience. But for a smaller group, infections keep coming back. The clinical threshold for recurrent yeast infections is four or more episodes in a single year, or three or more episodes that aren’t related to antibiotic use. Fewer than 5% of women fall into this category.
Recurrent infections sometimes signal an underlying issue worth investigating, such as undiagnosed diabetes, a resistant Candida strain, or a chronic immune challenge. They can also result from a cycle where treatment clears the symptoms but doesn’t fully eliminate the yeast, allowing it to rebound once conditions shift again. If you’re experiencing frequent infections, a healthcare provider can culture the yeast to identify the exact species, since non-albicans strains are harder to treat with standard over-the-counter options.