Yeast infections are caused by an overgrowth of a fungus called Candida that naturally lives on your skin and inside your body. Under normal conditions, Candida exists in small amounts without causing problems. But when something disrupts the balance of microorganisms that keep it in check, Candida multiplies rapidly and triggers infection. About 75% of women will experience at least one vaginal yeast infection in their lifetime, and 40% to 45% will have two or more.
The Fungus Behind the Infection
Candida albicans is the species responsible for most yeast infections, but it’s not the only one. The CDC lists five common species that can overgrow and cause problems: Candida albicans, Candida glabrata, Candida parapsilosis, Candida tropicalis, and Candida krusei. This matters because some non-albicans species are harder to treat. Candida glabrata, for example, is more often resistant to standard antifungal medications than other species, and roughly 5% of invasive Candida glabrata cases resist one or more antifungal drugs.
Candida lives in the mouth, gut, vagina, and on skin, particularly in warm, moist folds. It’s a normal part of your body’s ecosystem. The infection isn’t about catching Candida from somewhere; it’s about Candida that’s already there growing out of control.
How Your Body Normally Keeps Candida in Check
In the vagina, beneficial bacteria called Lactobacillus produce lactic acid that maintains an acidic environment. This acidity directly inhibits Candida growth. Not all Lactobacillus strains are equally protective, though. Research published in the American Journal of Obstetrics and Gynecology found that Lactobacillus crispatus produces significantly more lactic acid and inhibits Candida albicans growth more effectively than Lactobacillus iners. Women whose vaginal microbiome is dominated by L. crispatus appear to have stronger natural defenses against yeast overgrowth.
Your immune system also plays a constant role, identifying and clearing excess fungal cells before they can establish an infection. When either of these defenses falters, whether from medications, hormonal shifts, or illness, Candida seizes the opportunity.
Antibiotics Are a Leading Trigger
Antibiotics are one of the most common reasons yeast infections develop. Broad-spectrum antibiotics don’t just kill the bacteria causing your illness; they also wipe out the Lactobacillus and other beneficial bacteria keeping Candida in check. With those protective microbes depleted, Candida faces less competition and can multiply freely.
The damage goes beyond just clearing out good bacteria. Research in Cell Host & Microbe found that antibiotic-driven disruption of gut bacteria also impairs immune cells that produce key antifungal signals. Specifically, antibiotics reduced a type of immune cell in the gut that helps coordinate the body’s defense against fungal invasion. Even a single antibiotic class, like vancomycin, was enough to enhance susceptibility to fungal infection by disrupting this immune response. The effect only occurred when the antibiotics actually caused microbial imbalance, confirming that it’s the disruption of your body’s ecosystem, not a direct drug effect, that opens the door.
Hormonal Changes and Estrogen
Estrogen has a direct relationship with Candida growth, which is why yeast infections are more common during pregnancy, in the second half of the menstrual cycle, and among people using hormonal contraceptives or hormone replacement therapy. Elevated estrogen levels influence the expression of genes involved in Candida’s ability to shift into its more invasive, filament-forming state. These filaments help the fungus penetrate tissue and establish infection.
Estrogen also increases the glycogen content of vaginal cells, which provides more fuel for yeast to feed on. This combination of enhanced Candida virulence and a richer food supply explains why hormonal fluctuations are such a reliable trigger. Pregnant women, who sustain high estrogen levels for months, are particularly susceptible.
High Blood Sugar and Diabetes
Persistently elevated blood sugar creates ideal conditions for yeast. Candida thrives on glucose, and when there’s more of it circulating through your tissues and bodily fluids, the fungus grows faster. People with poorly controlled diabetes are at significantly higher risk for yeast infections in multiple locations. In the mouth, saliva with high sugar content directly promotes Candida overgrowth, which is one reason oral thrush is a recognized complication of uncontrolled diabetes.
Research from Penn State has also shown that high fasting blood glucose is linked to shifts in the fungal communities living in the gut, suggesting that elevated blood sugar doesn’t just feed existing Candida but may reshape your body’s entire fungal ecosystem in ways that favor overgrowth.
Clothing, Moisture, and Hygiene Habits
Candida thrives in warm, moist environments, which makes everyday choices about clothing and hygiene surprisingly relevant. Synthetic underwear traps heat and moisture against the skin, creating conditions where yeast flourishes. Cotton underwear wicks away excess sweat and moisture, and the Cleveland Clinic notes that even synthetic underwear with a cotton crotch panel doesn’t fully protect you because the surrounding fabric still limits airflow. Going without underwear at night, or wearing loose-fitting pajamas, increases ventilation and can help if you’re prone to recurrent infections.
Douching is another significant risk factor. The Office on Women’s Health warns that douching disrupts the natural balance of vaginal bacteria and acidity, which can directly cause yeast overgrowth. A healthy vagina maintains its own acidic environment that keeps infections at bay. Introducing water, vinegar, or commercial douching products strips away that protection. Scented tampons, pads, powders, and sprays carry similar risks by irritating the vaginal lining and altering its microbial balance.
Weakened Immune System
Any condition or treatment that suppresses your immune system raises the risk of yeast infections. This includes HIV/AIDS, chemotherapy, organ transplant medications, and long-term corticosteroid use. Your immune system is the second line of defense after beneficial bacteria, and when it’s compromised, Candida can overgrow not just on skin and mucous membranes but potentially spread into the bloodstream in severe cases.
Yeast Infections Beyond the Vagina
While vaginal yeast infections get the most attention, Candida causes infections in several other parts of the body, each with its own set of triggers.
Oral Thrush
Oral thrush appears as creamy white patches on the tongue, inner cheeks, and sometimes the roof of the mouth, gums, or tonsils. The patches look slightly raised, sometimes compared to cottage cheese, and can cause redness, burning, cracking at the corners of the mouth, and loss of taste. Wearing dentures (especially upper dentures), having dry mouth, using inhaled corticosteroids for asthma, and uncontrolled diabetes all increase the risk. Infants and older adults are more commonly affected because of their developing or declining immune function.
Skin Fold Infections
Candida also causes infections in skin folds: under the breasts, in the groin, between fingers, and in the armpits. These areas stay warm and moist, especially in hot weather or with tight clothing, giving Candida the conditions it needs. The resulting rash is typically red, itchy, and may have small satellite spots around the edges. People who are overweight or who sweat heavily are more prone to these infections because they have deeper skin folds that retain moisture.
Why Some People Get Recurring Infections
For some people, yeast infections come back repeatedly, often four or more times a year. Recurrent infections can happen because the underlying trigger is ongoing, such as uncontrolled blood sugar, continuous hormonal contraceptive use, or chronic immune suppression. In other cases, the species involved may be harder to eliminate. Candida glabrata and other non-albicans species are naturally less responsive to the most commonly used antifungal treatments, so infections caused by these species may not fully clear and can return. About 6% of Candida infections overall are resistant to the standard first-line antifungal, and resistance rates are climbing for some species in certain regions.
If you’re experiencing frequent yeast infections, identifying and addressing the underlying cause, whether that’s switching medications, improving blood sugar control, or changing hygiene habits, is more effective long-term than treating each episode individually.