Yeast infections happen when a fungus called Candida, which naturally lives in the vagina in small amounts, 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 overgrowth isn’t usually caused by something you “caught” from the outside. It’s almost always triggered by a shift in your body’s internal environment that lets existing yeast multiply.
The Balance That Keeps Yeast in Check
A healthy vagina is home to a community of beneficial bacteria, primarily Lactobacillus species, that produce lactic acid and keep the vaginal pH below 4.5. That acidic environment does two things: it directly inhibits yeast growth, and it prevents Candida from shifting into a more aggressive form that can invade tissue and cause inflammation. When something disrupts this bacterial balance or weakens your immune defenses, Candida seizes the opportunity.
The yeast itself is almost always already present. Your gastrointestinal tract is the main reservoir, and Candida colonizes the vaginal area from there early in life. So the real question isn’t “where did I pick this up?” but “what changed inside my body to let it overgrow?”
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics are one of the most well-established causes of yeast infections. These drugs kill the bacteria responsible for whatever infection you’re treating, but they also wipe out beneficial Lactobacillus in the vagina. Without those protective bacteria holding the line, Candida multiplies freely. Not every antibiotic carries this risk equally. Narrow-spectrum antibiotics, which target a smaller range of bacteria, are less likely to cause problems. But if you’ve been prescribed a broad-spectrum course for something like a sinus infection, urinary tract infection, or respiratory illness, a yeast infection afterward is a common side effect.
Hormones and Estrogen
High estrogen levels increase your risk because estrogen stimulates vaginal cells to produce more glycogen, a type of sugar stored in tissue. Candida albicans, the most common species behind yeast infections, feeds on glycogen directly. In lab studies, all 34 tested strains of C. albicans were able to break down and use glycogen as a food source, while most non-albicans Candida species could not. This is one reason the most common yeast species is also the most problematic.
This estrogen connection explains why yeast infections spike during pregnancy, when estrogen levels rise dramatically. It also explains why some women on hormonal birth control, particularly estrogen-containing pills, notice more frequent infections. Research in postmenopausal women found a strong link between estrogen use and Candida infections, reinforcing that the hormone itself, not just age or lifestyle, drives the risk.
Blood Sugar and Diabetes
Yeast thrives on sugar, and elevated blood glucose gives it exactly what it needs. When blood sugar runs high, excess sugar can be excreted through urine, creating a sugar-rich environment in the genital area that encourages both yeast and bacteria to grow. Women with uncontrolled diabetes are at notably higher risk. Once blood sugar is brought under control, infection frequency typically drops. This also raises the question of whether dietary sugar plays a role for people without diabetes, and while some healthcare providers suggest reducing sugar intake as part of prevention, the evidence for that link in otherwise healthy people is less clear-cut than it is for those with diabetes.
Sex Can Trigger It, but It’s Not an STI
Yeast infections are not classified as sexually transmitted infections, but sexual activity can play a role. Studies have found identical strains of Candida on the genitals of male partners of women with recurrent infections, confirming that the fungus can transfer between partners during sex. However, sexual transfer doesn’t appear to be the main way vaginal colonization happens. More often, sex acts as a trigger for yeast that’s already present, promoting overgrowth and inflammation rather than introducing new organisms.
This distinction matters practically. Clinical trials of treating male partners with antifungal medication have not shown a benefit in preventing women’s recurrent infections. Current guidelines don’t recommend routine partner treatment, since long-term antifungal regimens for the person experiencing symptoms are more effective at prevention.
Clothing and Moisture
Yeast grows best in warm, moist environments, so anything that traps heat and moisture against the vulva can contribute. Synthetic underwear is the main culprit. Cotton is breathable and wicks away sweat and vaginal moisture that yeast feeds on. A synthetic pair with a small cotton crotch panel doesn’t offer the same protection, because the surrounding synthetic fabric still limits airflow.
A few practical habits lower the risk: change out of wet swimsuits or sweaty workout clothes promptly, consider skipping underwear at night to increase airflow (loose pajamas or boxer shorts work well), and avoid wearing panty liners all day unless you need them for incontinence or your period, since they reduce breathability and can cause irritation.
Douching and Feminine Hygiene Products
Douching is one of the worst things you can do for vaginal health, and this applies to every type of douche, including plain water. Research shows that even water douches temporarily wash out protective Lactobacillus bacteria. Vinegar douches are equally harmful. Although Lactobacillus naturally produces acid, the lactic acid it makes is not interchangeable with acetic acid (vinegar). The two have different effects on the vaginal ecosystem, and vinegar cannot replicate the protective role of the bacteria it washes away.
Scented soaps, sprays, and washes applied to the vaginal area can similarly disrupt the microbial balance. The vagina is self-cleaning, and external washing of the vulva with mild, unscented soap is all that’s needed.
When Infections Keep Coming Back
Recurrent vulvovaginal candidiasis is defined as four or more infections within a single year. About 5% to 8% of women experience this pattern. In some cases, the immune system’s response to Candida gets stuck in a cycle: the yeast triggers inflammation, immune cells rush to the area but can’t effectively clear the fungus in its aggressive form, and the inflammatory response keeps going without resolving the underlying problem.
If you’re dealing with frequent infections, it’s worth looking at the risk factors above systematically. Are you on antibiotics often? Is your blood sugar well controlled? Are you using products that disrupt vaginal flora? Sometimes the trigger is identifiable and fixable. When it’s not, long-term suppressive antifungal treatment is effective at keeping recurrences at bay.