Yeast infections happen when a fungus called Candida, which normally lives on your skin and inside your body in small amounts, multiplies beyond what your immune system and local bacteria can keep in check. Between 29% and 49% of women will experience at least one diagnosed vaginal yeast infection in their lifetime, and the triggers range from antibiotics to hormonal shifts to something as simple as the underwear you wear.
How Candida Goes From Harmless to Harmful
Candida lives in the vaginal tract, the gut, the mouth, and on the skin of most healthy people without causing any problems. It stays in a round, inactive yeast form that your body tolerates. The trouble starts when something disrupts the environment that keeps it contained. Once conditions shift in Candida’s favor, the organism transforms from that harmless round shape into long, branching filaments called hyphae. These filaments can physically penetrate the cells lining your vaginal wall or skin, triggering inflammation and the classic symptoms: itching, burning, thick white discharge, and redness.
This shape-shift also lets the fungus produce proteins that help it grip tightly to tissue and a toxin called candidalysin that damages cells directly. Your immune system responds by flooding the area with inflammatory signals and sending in white blood cells, which is what causes the swelling and soreness. In other words, much of the discomfort you feel during a yeast infection is your own immune response fighting back.
The Protective Role of Lactobacillus Bacteria
The single biggest factor keeping vaginal yeast in check is a group of bacteria called Lactobacillus. These bacteria produce lactic acid, which holds vaginal pH between 3.5 and 4.5, an acidity level that directly prevents Candida from making that dangerous shape-shift into its invasive form. Lactobacillus species also compete with yeast for space on the vaginal lining, physically blocking Candida from attaching to cells. On top of that, they release hydrogen peroxide, short-chain fatty acids, and other antimicrobial compounds that suppress yeast growth.
Not all Lactobacillus species are equally protective. A vaginal environment dominated by L. crispatus tends to be the most resistant to yeast infections. Interestingly, when a different species called L. iners dominates instead, the environment becomes more inflammatory and actually encourages Candida to form filaments and biofilms, increasing infection risk.
Why Antibiotics Are a Common Trigger
Antibiotics are one of the most well-known causes of yeast infections, and the mechanism is straightforward. Antibiotics kill bacteria, including the protective Lactobacillus in the vaginal tract. When those populations drop, lactic acid production falls, vaginal pH rises, and Candida is free to multiply and shift into its invasive form. The yeast itself is a fungus, so antibiotics don’t touch it. You’re essentially removing its competition while leaving it completely intact.
This doesn’t happen with every antibiotic course, but broad-spectrum antibiotics (the kind prescribed for sinus infections, urinary tract infections, or respiratory infections) are the most likely to cause it because they wipe out a wider range of bacteria.
Hormones, Pregnancy, and Birth Control
Estrogen plays a direct role in yeast infection risk. Higher estrogen levels increase the amount of glycogen (a sugar stored in tissue) deposited in vaginal cells. Lactobacillus bacteria feed on this glycogen to produce lactic acid, which sounds protective. But when estrogen spikes significantly, the excess glycogen can also feed Candida, tipping the balance.
This is why yeast infections are more common during pregnancy, when estrogen levels are dramatically elevated. It also explains why some people notice more infections after starting hormonal birth control or hormone replacement therapy. The luteal phase of your menstrual cycle (the two weeks before your period) also brings a slight estrogen and progesterone shift that can make some people more susceptible.
Blood Sugar and Diabetes
Elevated blood sugar creates a more hospitable environment for Candida in a very specific way. When glucose levels rise in vaginal tissue, the cells lining the vaginal wall increase production of a surface molecule called ICAM-1. This molecule essentially acts as a docking station, giving Candida something extra to grip onto. Research on vaginal epithelial cells cultured in high-glucose conditions showed significantly increased Candida adhesion compared to normal glucose levels.
This is why people with poorly controlled diabetes, both type 1 and type 2, experience yeast infections at higher rates. Certain diabetes medications that work by pushing excess sugar out through urine can further raise the glucose concentration in the genital area, compounding the problem. If you get frequent yeast infections and haven’t had your blood sugar checked, it’s worth mentioning to your doctor.
Clothing, Moisture, and Hygiene Products
Candida thrives in warm, moist environments. Anything that traps heat and moisture against your skin creates conditions the fungus loves. Cotton underwear is the most protective choice because it wicks away sweat and allows airflow. Synthetic fabrics, even those with a small cotton crotch panel, don’t breathe the same way and can keep moisture locked against the skin.
A few practical changes that reduce risk:
- Skip panty liners when you don’t need them. Wearing them daily decreases breathability and can cause irritation that makes infection more likely.
- Sleep without underwear or in loose pajamas. Increased airflow overnight helps keep the area dry, which is especially useful if you’re prone to recurring infections.
- Use fragrance-free, dye-free detergent. Scented detergents leave residue on fabric that can irritate vulvar skin, weakening your local defenses.
- Change out of wet swimsuits or workout clothes promptly. Prolonged moisture exposure is one of the most avoidable triggers.
Douching and scented vaginal products also disrupt the vaginal microbiome, killing off protective Lactobacillus and raising pH. The vagina is self-cleaning; external washing with plain water is all that’s needed.
Weakened Immune System
Your immune system normally keeps Candida in its harmless yeast form. When immune function is compromised, whether from HIV, chemotherapy, organ transplant medications, or chronic stress, the body loses its ability to suppress Candida’s transition into its invasive form. People with weakened immune systems are also more likely to develop yeast infections in the mouth (oral thrush), the esophagus, or even the bloodstream, not just the genital area.
Men Get Yeast Infections Too
Yeast infections aren’t exclusive to people with vaginas. Men can develop a Candida infection on the penis called balanitis, which causes redness, itching, and sometimes a white, patchy coating on the head of the penis. The risk factors overlap significantly with those for vaginal infections: diabetes, long-term antibiotic use, a weakened immune system, and excess moisture.
Being uncircumcised increases the risk because the foreskin creates a warm, moist fold of skin where yeast can multiply. Excess weight can contribute for the same reason, creating skin folds that trap moisture. Sexual transmission is also possible: having sex with a partner who has an active vaginal yeast infection raises your chances of developing one.
Recurrent Infections
Most people who get a yeast infection deal with it once and move on. But for a meaningful minority, infections keep coming back. Recurrent vulvovaginal candidiasis is defined as three or more episodes within a single year and affects fewer than 5% of women. Among those who experience even one yeast infection, though, more than one in five will eventually have a stretch where infections recur four or more times in 12 months.
Recurrence often points to an underlying factor that hasn’t been addressed: uncontrolled blood sugar, a microbiome that never fully recovers its Lactobacillus populations, ongoing hormonal influences, or a Candida strain that’s partially resistant to standard antifungal treatment. For people dealing with recurrence, newer antifungal options have expanded the available treatments beyond the familiar over-the-counter creams and single-dose pills, giving clinicians more tools for long-term management.