Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, grows out of control. An estimated 75% of women will have at least one yeast infection in their lifetime, and 40% to 45% will have two or more. The causes range from antibiotics and hormonal shifts to everyday habits that disrupt the vaginal environment.
How the Vagina Normally Keeps Yeast in Check
A healthy vagina maintains a pH between 3.8 and 4.5, which is acidic enough to suppress yeast growth. Beneficial bacteria, primarily Lactobacillus species, are responsible for this acidity. They produce lactic acid, hydrogen peroxide, and other compounds that limit Candida from multiplying or changing into its more aggressive form.
Lactobacillus bacteria also compete directly with yeast for nutrients and attachment sites on vaginal tissue. They strengthen the vaginal lining by boosting the proteins that hold cells tightly together, making it harder for yeast to invade. When something disrupts this bacterial population or changes the vaginal environment, Candida can shift from a harmless round yeast form into elongated filaments that burrow into the vaginal lining, release a damaging toxin, and trigger the inflammation you feel as itching, burning, and discharge.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics are designed to kill a wide range of bacteria, but they don’t distinguish between harmful bacteria and the protective Lactobacillus in your vagina. When those beneficial bacteria are wiped out, there’s nothing keeping Candida in check. The yeast rapidly fills the gap, and symptoms can appear within days of starting an antibiotic course. This is one of the most well-established causes of yeast infections, and it’s why many women develop one during or shortly after treatment for something entirely unrelated, like a sinus infection or urinary tract infection.
Hormonal Changes and Estrogen
Estrogen plays a direct role in shaping the vaginal environment. It stimulates the vaginal lining to produce glycogen, a type of stored sugar that feeds Lactobacillus bacteria. This sounds protective, and in moderate amounts it is. But when estrogen levels spike, the extra glycogen can also fuel Candida growth, tipping the balance.
This is why yeast infections are more common during pregnancy, in the second half of the menstrual cycle, and in women using higher-estrogen birth control. On the flip side, the drop in estrogen after menopause thins the vaginal lining, reduces glycogen, and shifts pH to a more alkaline range of 6 to 8. That different environment makes yeast infections less common but increases vulnerability to other types of vaginal infections.
High Blood Sugar Feeds Yeast Directly
Candida thrives in sugar-rich environments. When blood sugar is consistently elevated, as in uncontrolled type 2 diabetes, glucose levels rise throughout the body’s tissues, including vaginal secretions. That excess sugar acts as a preferred energy source for yeast, helping it survive, colonize, and adhere more aggressively to vaginal cells. Women with poorly managed diabetes are significantly more prone to recurrent yeast infections, and getting blood sugar under control is often a key part of breaking the cycle.
Even without diabetes, a diet very high in simple sugars and refined carbohydrates may contribute to recurring infections. Yeast feeds on sugar, and reducing intake of white flour, white rice, and sugary foods has been associated with fewer or less severe episodes. You likely don’t need to eliminate these foods entirely to see a difference.
A Weakened Immune System
Your immune system is what ultimately clears a yeast overgrowth. When Candida begins damaging vaginal tissue, the body launches an inflammatory response, recruiting immune cells to the site to kill the fungus. If that immune response is weakened, yeast can grow unchecked.
Several conditions and treatments suppress the immune system enough to raise yeast infection risk. HIV/AIDS is the most significant: esophageal candidiasis is one of the most common infections in people living with HIV. Cancer, particularly blood cancers like leukemia and lymphoma, also increases susceptibility. Medications that dampen immune function, including corticosteroids and chemotherapy drugs, have the same effect. Recurrent yeast infections that don’t respond to standard treatment can sometimes be an early signal of an undiagnosed immune condition.
Douching and Chemical Irritants
The vagina is self-cleaning, and douching disrupts that process. Flushing the vaginal canal with water, vinegar, or commercial douching products strips away normal bacteria and alters the natural acidity that keeps yeast in check. The U.S. Office on Women’s Health notes that douching can directly cause an overgrowth of harmful organisms, leading to yeast infections or bacterial vaginosis.
Scented products pose a similar risk. Fragranced tampons, pads, powders, and sprays can irritate vaginal tissue and shift the microbial balance. Even mild soaps can cause dryness and irritation if you have sensitive skin or an existing infection. Washing the external vulva with warm water, or at most an unscented gentle cleanser, is all that’s needed.
Moisture and Clothing Choices
Yeast grows best in warm, moist environments, which is why everyday clothing choices can matter. Synthetic fabrics trap heat and moisture against the skin, creating conditions that encourage Candida to multiply. Cotton underwear is the better option because it wicks away sweat and allows airflow. Underwear with a small cotton crotch panel sewn into synthetic fabric doesn’t offer the same protection, since the surrounding material still traps moisture.
Sitting in a wet swimsuit, wearing tight leggings for extended periods, or sleeping in snug underwear all reduce ventilation. Going without underwear at night, or switching to loose pajamas or boxer shorts, increases airflow and can help prevent infections or speed healing during an active one.
Recurrent Yeast Infections
Most women who get a yeast infection have an isolated episode that clears with treatment. But fewer than 5% of women experience recurrent infections, defined as three or more symptomatic episodes in a single year. Recurrent infections often involve the same underlying causes described above, layered on top of each other: a woman with mildly elevated blood sugar who also takes frequent antibiotics and wears non-breathable clothing, for example, faces compounding risk.
It’s also worth noting that not every episode of itching or unusual discharge is a yeast infection. Bacterial vaginosis, which involves a different type of microbial imbalance, produces similar symptoms but requires different treatment. One distinguishing clue is vaginal pH: yeast infections typically occur at a normal acidic pH (under 4.5), while bacterial vaginosis pushes pH higher. Self-treating repeatedly with over-the-counter antifungal creams without a confirmed diagnosis can mask other conditions and delay appropriate care.