Yeast infections happen when a fungus called Candida, which normally lives in the vagina in small amounts, grows out of control. About 75% of women will experience at least one vaginal yeast infection in their lifetime, and 5 to 10% develop recurrent infections, defined as four or more episodes per year. The shift from harmless presence to active infection comes down to a disruption in the balance between the fungus, the vaginal microbiome, and your immune system.
How Candida Goes From Harmless to Harmful
Candida albicans is the species behind most vaginal yeast infections. It lives on mucosal surfaces throughout your body, including the vagina, mouth, and gut, without causing problems. In its dormant form, it exists as round yeast cells. But when conditions change, it can switch into an aggressive form, growing long filaments called hyphae that penetrate tissue and trigger inflammation. This yeast-to-hyphae transition is the key moment an infection begins.
What keeps Candida in check under normal circumstances is a three-way balance. Beneficial bacteria (primarily Lactobacillus species) dominate the vaginal microbiome and produce lactic acid, maintaining a naturally acidic environment with a pH between 3.8 and 4.5. That acidity suppresses Candida growth. Your immune system also actively patrols the area, producing antimicrobial compounds and recruiting white blood cells to fight fungal overgrowth. When either of these defenses weakens, or when something shifts the local environment in Candida’s favor, the fungus seizes the opportunity.
Antibiotics Are the Most Common Trigger
Taking antibiotics is one of the most well-established causes of yeast infections. Antibiotics kill bacteria indiscriminately, wiping out the protective Lactobacillus alongside whatever infection you’re treating. With fewer beneficial bacteria producing acid and competing for space, Candida faces less resistance and can multiply rapidly. Broad-spectrum antibiotics pose the highest risk because they affect the widest range of bacterial species, but any antibiotic course can tip the balance.
This is why yeast infections so often follow a round of antibiotics for a urinary tract infection, sinus infection, or strep throat. The yeast infection isn’t a side effect of the drug itself. It’s the consequence of losing the bacterial community that was keeping Candida suppressed.
Hormonal Changes and Estrogen Levels
Estrogen promotes the growth of Lactobacillus, but it also increases glycogen (a sugar) in vaginal cells, which Candida feeds on. This is why yeast infections cluster around specific hormonal events. Pregnancy raises estrogen significantly, making yeast infections more common throughout all three trimesters. The luteal phase of the menstrual cycle (the week or so before your period) also brings a brief estrogen spike that some women notice correlates with symptoms.
Hormonal birth control, particularly high-estrogen formulations, can increase susceptibility for the same reason. Hormone replacement therapy during menopause can also shift the balance. The pattern is consistent: higher estrogen means more fuel for Candida, even though it simultaneously supports the bacteria that fight it.
A Weakened Immune System
Your immune system uses a specific branch of defense to fight Candida at mucosal surfaces. Specialized immune cells produce signaling molecules that recruit neutrophils (a type of white blood cell) and trigger the release of natural antimicrobial proteins. Research in the Journal of Experimental Medicine showed that when this pathway is disrupted in animal models, neutrophil recruitment drops sharply and the body’s natural antifungal compounds decrease by as much as sixfold, leading to severe fungal overgrowth.
Anything that suppresses immune function can open the door to yeast infections. HIV and AIDS are the most dramatic examples, but immunosuppressive medications taken after organ transplants or for autoimmune conditions also increase risk. Chronic stress and sleep deprivation subtly weaken immune surveillance over time. Even a bad cold won’t directly cause a yeast infection, but the immune system is working with finite resources, and diverting them elsewhere can leave mucosal defenses thinner.
Uncontrolled Blood Sugar and Diabetes
Candida thrives on sugar. When blood glucose levels run consistently high, as in uncontrolled type 1 or type 2 diabetes, sugar concentrations in vaginal secretions rise as well. This provides the fungus with abundant fuel. Women with diabetes who have difficulty managing their blood sugar experience yeast infections at significantly higher rates than women with well-controlled glucose levels.
You don’t need a diabetes diagnosis for this to matter. Diets very high in refined sugar and simple carbohydrates may contribute to Candida-friendly conditions, though the evidence here is less definitive than it is for clinical hyperglycemia. If you’re getting frequent yeast infections without another obvious cause, it’s worth having your blood sugar checked.
Moisture, Clothing, and Local Environment
Candida grows best in warm, moist environments. Sitting in a wet swimsuit or sweaty workout clothes for hours creates exactly those conditions. Synthetic fabrics like nylon and spandex trap heat and moisture against the skin rather than allowing airflow, which can encourage yeast growth.
That said, the evidence on clothing choices is less dramatic than you might expect. A study examining whether underwear style affected the vulvar microenvironment found no measurable changes in pH, skin moisture levels, or bacterial flora based on underwear type. The bigger factor is prolonged exposure to excess moisture rather than the specific garment. Changing out of wet or sweaty clothing promptly matters more than whether your underwear is cotton or synthetic.
Douching and Irritating Products
The vagina is self-cleaning. Douching, scented soaps, vaginal deodorants, and perfumed bath products disrupt the natural microbial balance by altering pH and killing beneficial bacteria. The effect is similar to antibiotics: you remove the organisms keeping Candida in check. Douching in particular has been repeatedly linked to higher rates of both yeast infections and bacterial vaginosis.
Bubble baths, scented tampons, and even some laundry detergents can cause enough local irritation to compromise the mucosal barrier, making it easier for Candida to transition to its invasive form. Washing the external vulva with warm water, or at most a mild unscented cleanser, is all that’s needed.
Sexual Activity
Yeast infections are not sexually transmitted infections, but sexual activity can play a role. Intercourse can introduce new microorganisms, alter vaginal pH, and cause minor tissue irritation that changes the local environment. Saliva, lubricants, and spermicides can all shift the chemical balance. Some women notice a pattern of yeast infections after sex with a new partner as their microbiome adjusts.
Oral sex can transfer Candida from the mouth to the genitals. In rare cases, a partner with a penile yeast infection can pass the fungus back and forth, creating a cycle of reinfection. This is uncommon but worth considering if infections keep returning despite treatment.
Why Some Women Get Recurrent Infections
For the 5 to 10% of women who experience four or more yeast infections per year, the causes often overlap or resist easy identification. More than 50% of women receiving maintenance therapy for recurrent infections experience another episode after stopping treatment, suggesting that some individuals have an underlying susceptibility that standard antifungal courses don’t resolve.
Genetic differences in immune signaling may explain part of this. Some women produce fewer of the antimicrobial compounds that keep Candida dormant, making them perpetually closer to the threshold where overgrowth begins. Non-albicans Candida species, particularly Candida glabrata, are increasingly recognized in recurrent cases. These species are harder to treat because they more frequently resist standard antifungal medications. If your infections keep coming back despite treatment, your doctor may culture the specific species involved to guide therapy.
Recurrent infections can also stem from persistent, low-level triggers that are easy to overlook: ongoing antibiotic use for another condition, mildly elevated blood sugar that hasn’t been flagged, or chronic stress that keeps immune defenses slightly suppressed. Identifying and addressing these underlying factors is often more effective than treating each episode in isolation.