Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, grows out of control. About 75% of women will get at least one vaginal yeast infection in their lifetime, and 40% to 45% will deal with two or more. The fungus is almost always already there. The real question is what tips the balance.
What Keeps Yeast in Check
Your vagina hosts a community of bacteria that act as a natural defense system. The most important players are Lactobacillus bacteria, which produce lactic acid and keep vaginal pH between 4 and 4.5. That acidic environment limits the ability of Candida to grow and change shape. Lactobacillus bacteria also compete with yeast for nutrients and attachment sites on vaginal tissue, produce hydrogen peroxide and other antimicrobial compounds, and strengthen the physical barrier of vaginal cells by boosting the proteins that hold cells tightly together.
When this bacterial ecosystem gets disrupted, the pH rises, and Candida seizes the opportunity. The fungus shifts from a harmless round yeast form into elongated filaments that can physically invade the vaginal lining. Those filaments release a toxin that damages cells and triggers the inflammatory immune response you feel as itching, burning, and swelling.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics, the kind prescribed for sinus infections, bronchitis, or urinary tract infections, don’t just kill the bacteria making you sick. They wipe out Lactobacillus along with everything else. Without those protective bacteria, your vagina becomes less acidic and far more hospitable to yeast. You can develop a yeast infection during or shortly after finishing a course of antibiotics.
This doesn’t mean you should skip antibiotics when you need them. But if you’ve noticed a pattern of yeast infections following antibiotic use, that connection is real and well established.
Hormonal Changes That Fuel Yeast Growth
Estrogen plays a direct role in how your vaginal environment operates. Higher estrogen levels increase glycogen (a type of sugar) in vaginal tissue, which changes the local ecology in ways that can favor Candida. This is why yeast infections are more common during pregnancy, when estrogen levels are significantly elevated. Hormonal contraceptives and hormone replacement therapy can have a similar effect.
The pattern also explains why yeast infections are rare before puberty and after menopause, when estrogen levels are naturally lower. Some women notice infections clustering around certain points in their menstrual cycle, which tracks with normal estrogen fluctuations.
High Blood Sugar Creates a Feeding Ground
Women with diabetes have a higher risk of yeast infections, particularly when blood sugar is poorly controlled. The mechanism is straightforward: when blood sugar runs high, excess sugar can end up in vaginal secretions and urine, giving yeast an abundant food source. Keeping blood sugar well managed significantly reduces this risk.
You don’t need a diabetes diagnosis for this to matter. Any condition or medication that raises blood sugar levels can create a similar environment.
Clothing, Moisture, and Everyday Habits
Yeast thrives in warm, moist environments. Certain everyday choices can create exactly those conditions. Tight-fitting clothing and synthetic fabrics trap heat and moisture against the vulva, which encourages yeast growth. Cotton underwear is the best option because it wicks away sweat and allows airflow. Synthetic underwear with a small cotton crotch panel doesn’t offer the same protection, because the surrounding synthetic fabric still limits breathability.
Other habits that can shift conditions in yeast’s favor include sitting in a wet swimsuit for extended periods, using scented soaps or douches in the vaginal area (which disrupts pH and bacterial balance), and wearing tight workout clothes long after you’ve finished exercising. Looser, breathable clothing is particularly worth considering if you’re someone who gets frequent infections.
Sex, Partners, and Transmission
A yeast infection is not a sexually transmitted infection. You can get one without any sexual contact at all. That said, sex isn’t completely irrelevant. Friction can irritate vaginal tissue, and introducing new bacteria or lubricants can alter the vaginal environment. Some women notice infections tend to follow sexual activity.
If your partner is male, his risk of developing symptoms is low. About 15% of men get an itchy rash on the penis after unprotected sex with a woman who has an active yeast infection. If your partner is female, she may be at higher risk and should watch for symptoms.
Why Some People Get Them Over and Over
Recurrent yeast infections, typically defined as four or more episodes in a single year, affect a meaningful number of women. The causes overlap with the triggers above but tend to compound. Someone with a combination of risk factors (frequent antibiotic use, hormonal contraception, high blood sugar, or a naturally less diverse vaginal microbiome) faces a much higher cumulative risk than someone with just one.
Some women have vaginal microbiomes that are naturally less dominated by protective Lactobacillus species. Research has identified five distinct community types in the vaginal microbiome, and not all offer the same level of resistance to Candida overgrowth. A microbiome dominated by diverse anaerobic bacteria rather than Lactobacillus is associated with more frequent disruptions.
A weakened immune system, whether from a medical condition, medication, or chronic stress, also makes it harder for your body to keep Candida in its harmless form. The immune system normally recognizes early signs of fungal invasion and recruits cells to clear the infection. When that response is sluggish or impaired, yeast gains a foothold more easily.
What a Yeast Infection Feels Like
The most recognizable symptoms are intense itching and irritation around the vulva and vaginal opening, a thick white discharge often described as resembling cottage cheese, burning during urination, and pain or discomfort during sex. Not every yeast infection causes all of these, and mild infections sometimes produce only itching.
These symptoms overlap with other conditions, including bacterial vaginosis and certain STIs. If you’ve never had a yeast infection before, or if your symptoms don’t match your previous experiences, getting an accurate diagnosis matters. Over-the-counter antifungal treatments work well for straightforward yeast infections, but they won’t help if the actual cause is something else.