Most vaginal yeast infections happen when a fungus called Candida, which normally lives in small amounts in 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, and 40% to 45% will have two or more. The infection isn’t usually caused by exposure to something new. It’s caused by a shift in the environment that already exists inside your body.
The Balance That Keeps Yeast in Check
A healthy vagina is home to a community of bacteria dominated by Lactobacillus species. These bacteria produce lactic acid, which keeps vaginal pH between 4 and 4.5. At that acidity level, Candida stays in a harmless, rounded cell form and can’t easily switch into the elongated, invasive form that causes symptoms. Lactobacillus crispatus is particularly protective because it produces a type of lactic acid (d-lactic acid) that is especially effective at suppressing fungal growth.
Beyond just lowering pH, these bacteria fight Candida in several other ways. They produce hydrogen peroxide and antimicrobial peptides. They physically compete with yeast for space on the vaginal lining. They also strengthen the barrier of cells lining the vaginal wall by boosting the proteins that hold those cells tightly together, making it harder for yeast to penetrate tissue. When something disrupts this bacterial community or weakens any of these defenses, Candida can overgrow and cause infection.
Antibiotics Are a Leading Trigger
Antibiotics kill bacteria, and they don’t distinguish between the ones causing your sinus infection and the Lactobacillus keeping your vagina healthy. When those protective bacteria are wiped out, the pH rises, lactic acid production drops, and Candida faces far less competition. This is why yeast infections commonly show up during or shortly after a course of antibiotics, even antibiotics taken for something completely unrelated to the vagina.
Hormonal Shifts and Estrogen
Estrogen plays a significant role in yeast infection risk because it increases the amount of glycogen (a type of sugar) stored in vaginal cells. More glycogen means more fuel for both Lactobacillus and Candida, but under certain hormonal conditions, the balance tips toward yeast. This is why yeast infections are more common during pregnancy, in the second half of the menstrual cycle, and in women using hormonal contraceptives with higher estrogen levels. Before puberty and after menopause, when estrogen is low, yeast infections are relatively rare.
High Blood Sugar Feeds Yeast Growth
Women with diabetes face a higher risk of yeast infections, particularly when blood sugar is poorly controlled. When blood glucose runs high, excess sugar can appear in vaginal secretions and urine, giving Candida an abundant food source. This doesn’t just apply to people with a diabetes diagnosis. Any period of elevated blood sugar, whether from undiagnosed diabetes, steroid medications, or other causes, can create conditions that favor yeast overgrowth.
Douching and Scented Products
Douching is one of the most direct ways to disrupt vaginal balance. It flushes out the protective bacteria that keep yeast in check, and the body’s attempt to repopulate those bacteria can lead to an overproduction cycle that ends in infection. The Cleveland Clinic advises against douching entirely, noting that putting any foreign substance into the vagina can cause irritation, remove healthy bacteria, and throw off pH balance.
Scented soaps, bubble baths, sprays, and scented tampons or pads pose a similar problem. They can irritate the vaginal lining and alter the microbial environment. Some over-the-counter feminine hygiene products can actually worsen symptoms or cause additional skin irritation.
Moisture, Heat, and Clothing Choices
Candida thrives in warm, moist environments. Underwear made from synthetic materials like nylon and spandex traps moisture and heat against the skin, creating conditions that encourage fungal growth. Tight-fitting clothing restricts airflow and compounds the problem. Cotton underwear is the best choice because it breathes and absorbs moisture from sweat and discharge more effectively than synthetics.
Sitting in a wet swimsuit or sweaty workout clothes for extended periods raises your risk for the same reason. If you’ve been particularly active or it’s a hot day and you notice extra moisture, changing into a fresh pair of underwear is a simple preventive step.
Sexual Activity and Yeast Infections
Yeast infections are not sexually transmitted infections, but sexual activity can contribute to getting one. Vaginal, oral, and anal sex, as well as fingering, can introduce new microbes or irritate vaginal tissue enough to shift the microbial balance. Semen has a higher pH than the vagina, so unprotected sex can temporarily raise vaginal pH, potentially giving Candida an opening. Using condoms or dental dams reduces this effect, and wiping away fluids afterward helps reduce lingering moisture.
A Weakened Immune System
Your immune system normally helps keep Candida from transitioning into its invasive form. Conditions that suppress immune function, such as HIV, chemotherapy, organ transplant medications, or long-term corticosteroid use, significantly increase the risk of yeast infections. Even temporary dips in immune function from stress, poor sleep, or illness can make an infection more likely, though these are harder to quantify.
How to Tell It’s a Yeast Infection
The hallmark symptom is a thick, white, cottage cheese-like discharge that typically has no strong odor. Itching, burning, redness, and swelling around the vulva are common, and you may feel pain or burning during urination or sex. These symptoms are often confused with bacterial vaginosis (BV), but the two conditions look different. BV tends to produce a thin, grayish discharge that is heavier in volume and has a noticeable fishy smell, especially after a period or intercourse. Yeast infections rarely have a strong odor.
This distinction matters because the treatments are completely different. Over-the-counter antifungal treatments work for most yeast infections caused by Candida albicans, the most common species. But if your symptoms don’t improve or keep coming back, a different Candida species may be involved, and some of these respond poorly to standard treatments. Recurrent infections, defined as three or more episodes in a single year, affect fewer than 5% of women and typically require a different management approach.
Multiple Factors Often Overlap
In practice, yeast infections rarely result from a single cause. A woman taking antibiotics who also wears synthetic underwear and is in the luteal phase of her cycle has three factors working against her at once. Understanding which contributors apply to your situation helps you reduce risk where you can: choosing breathable fabrics, avoiding douching, changing out of damp clothing promptly, and being aware that antibiotic courses may require some extra attention to vaginal health afterward.