Yeast infections happen when a fungus called Candida, which normally lives on your skin and mucous membranes without causing problems, grows out of control. 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 shift from harmless resident to infection-causing overgrowth comes down to changes in your body’s environment that give yeast an opening.
Your Body Already Has Yeast in It
Candida lives in small numbers on your skin, in your gut, and in the vagina as part of a balanced ecosystem. Under normal conditions, your immune system and the other microbes sharing that space keep it in check. The vagina, for example, is home to beneficial bacteria (mainly Lactobacillus species) that convert a sugar called glycogen into lactic acid. This lactic acid lowers vaginal pH, creating an acidic environment that suppresses the growth of opportunistic organisms, including yeast.
A yeast infection isn’t something you “catch” from the outside in most cases. It’s an internal shift. When something disrupts the balance of protective bacteria, changes the local chemistry, or weakens immune defenses, Candida cells already present can multiply rapidly and change form, growing thread-like structures that penetrate tissue and trigger inflammation.
Antibiotics Are a Common Trigger
Antibiotics are one of the most well-established causes of yeast infections because they don’t just kill the bacteria making you sick. They also wipe out beneficial Lactobacillus in the vagina. Without those acid-producing bacteria, vaginal pH rises, and the environment becomes far more hospitable for yeast. This is why many women develop a yeast infection during or shortly after a course of antibiotics for a sinus infection, urinary tract infection, or other condition that has nothing to do with the vagina itself.
The effect isn’t always temporary. Even after antibiotics are finished, a Lactobacillus-dominated microbiome doesn’t always bounce back on its own. The gap in bacterial protection can persist long enough for Candida to establish a foothold.
Hormonal Changes Shift the Balance
Estrogen plays a direct role in the vaginal environment. When estrogen levels rise, vaginal cells produce and store more glycogen, the sugar that feeds both Lactobacillus bacteria and, when conditions shift, Candida. This is why yeast infections are more common during certain windows of hormonal change.
Pregnancy brings a sustained surge in both estrogen and progesterone, increasing glycogen deposits on vaginal walls and making yeast overgrowth more likely. Hormonal contraceptives, particularly those with higher estrogen content, may also raise the risk. Some research links commonly used hormone-based birth control to changes in the vaginal microbiome that increase susceptibility to yeast infections, bacterial vaginosis, and genital inflammation.
The menstrual cycle matters too. Many women notice yeast infections tend to show up in the days just before their period, when progesterone peaks and then drops. After menopause, when estrogen levels fall significantly, vaginal yeast infections become less common, though they can still occur.
Blood Sugar Feeds the Problem
Yeast thrives on sugar. When blood sugar levels are consistently elevated, the glucose concentration in vaginal secretions rises as well, essentially providing extra fuel for Candida to grow. High blood sugar also shifts vaginal pH in a direction that favors yeast.
This is why women with poorly controlled diabetes are significantly more likely to develop yeast infections, and to have them recur. Managing blood sugar levels is one of the most effective ways to reduce that risk. Even in people without diabetes, a diet very high in simple sugars may contribute to more frequent infections. Cutting back on sugar, while not a guaranteed fix, has been associated with fewer and less severe episodes.
A Weakened Immune System Opens the Door
Your immune system is the primary gatekeeper preventing Candida from transitioning to an infectious state. Certain white blood cells recognize fungal cells, bind to them, and signal other immune cells to destroy them. When this system is compromised, the fungus faces far less resistance.
Conditions and treatments that suppress immunity create significant risk. HIV infection, chemotherapy, organ transplant medications, and long-term corticosteroid use all impair the specific immune pathways that hold Candida in check. People with these conditions may develop not only vaginal yeast infections but oral thrush or systemic fungal infections as well. Even everyday immune dips from chronic stress, poor sleep, or illness can tip the balance enough for a localized overgrowth.
Clothing and Moisture Create a Breeding Ground
Yeast grows best in warm, moist environments. Tight-fitting clothing made from synthetic fabrics like nylon and spandex traps heat and moisture against the skin rather than allowing it to evaporate. This creates exactly the conditions Candida needs to proliferate. Cotton underwear, by contrast, is breathable and absorbent, wicking moisture away from the body.
Sitting in a wet swimsuit for hours, wearing sweaty workout clothes long after exercising, or layering non-breathable fabrics in warm weather all raise the risk. Sleeping without underwear can help by letting the area air out overnight and preventing moisture from accumulating.
Douching and Scented Products Backfire
Douching, whether with water, vinegar, or commercial solutions, temporarily washes out the protective Lactobacillus bacteria that maintain vaginal acidity. Research has consistently shown that douching increases the risk of bacterial vaginosis, pelvic inflammatory disease, and yeast infections. No study has ever demonstrated a benefit to douching.
The logic that vinegar can substitute for the vagina’s natural acidity doesn’t hold up either. Lactic acid produced by vaginal bacteria and acetic acid from vinegar are not interchangeable. They have different antimicrobial properties, and rinsing with vinegar does not replicate the protection that a healthy bacterial community provides. Scented soaps, sprays, and wipes applied to the vulva or vagina can similarly disrupt the microbiome and irritate delicate tissue, lowering its defenses.
When Infections Keep Coming Back
Fewer than 5% of women experience recurrent yeast infections, defined as three or more symptomatic episodes in a single year. Recurrence often signals an underlying driver that hasn’t been addressed: uncontrolled blood sugar, ongoing antibiotic use, a hormonal factor, or an immune issue. In some cases, a less common strain of Candida is involved, one that doesn’t respond as well to standard antifungal treatments.
If you’re dealing with frequent infections, identifying and managing the root cause is more effective than treating each episode in isolation. Keeping blood sugar stable, choosing breathable fabrics, avoiding products that disrupt vaginal chemistry, and being aware of how antibiotics and hormonal medications affect your body are all practical steps that reduce the likelihood of another round.