Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, multiplies beyond what the body can keep in check. About 138 million women worldwide deal with recurrent yeast infections each year, defined as four or more episodes annually. The overgrowth isn’t usually caused by catching something new. It’s caused by a shift in the vaginal environment that lets a resident organism take over.
How the Vagina Normally Keeps Yeast Under Control
A healthy vagina maintains a pH between 4.0 and 4.5, which is moderately acidic. That acidity comes largely from beneficial bacteria called Lactobacillus, which produce lactic acid and other compounds as part of their normal metabolism. These bacteria do more than just lower pH. They physically compete with Candida for space on the vaginal lining, reduce the fungus’s ability to stick to tissue, and help activate immune defenses in the vaginal walls.
When this ecosystem is intact, Candida stays present but harmless. Most people carry it without symptoms. The problems start when something disrupts the balance, either by killing off protective bacteria, feeding the yeast extra fuel, or weakening the immune response that keeps fungal growth contained.
Antibiotics Are the Most Common Trigger
Antibiotics are designed to kill bacteria, but they don’t distinguish between harmful bacteria and the Lactobacillus that protect the vagina. In one large study, nearly 20% of women diagnosed with a yeast infection had used antibiotics in the previous month, compared to about 12% of women without an infection. No particular type of antibiotic carried a higher risk than another. Broad-spectrum and narrow-spectrum antibiotics both disrupted the vaginal environment.
The longer the course of antibiotics, the greater the risk. Women who took antibiotics for extended periods were significantly more likely to develop a yeast infection than those on short courses. And if you’ve had yeast infections before, antibiotics are more likely to trigger a recurrence. The pattern is straightforward: antibiotics reduce the bacterial population that holds Candida in check, and the fungus fills the gap.
Hormonal Changes That Feed Yeast Growth
Estrogen plays a direct role in yeast infections. Higher estrogen levels stimulate the cells lining the vagina to produce more glycogen, a form of stored sugar. Candida thrives on glycogen. Lab research confirms that all tested strains of the most common species can break down and use glycogen as a food source.
This explains why yeast infections are more common during certain life stages and situations:
- Pregnancy: Estrogen levels rise dramatically, especially in the second and third trimesters, creating a glycogen-rich environment that favors Candida.
- Hormonal contraceptives: Birth control pills, patches, and rings that contain estrogen can raise vaginal glycogen levels enough to tip the balance.
- Menstrual cycle fluctuations: Some women notice yeast infections clustering around certain points in their cycle, particularly when estrogen peaks before ovulation.
Before puberty and after menopause, when estrogen levels are naturally low, yeast infections are far less common. The connection between estrogen, glycogen, and Candida growth is one of the best-understood pathways in vaginal health.
Diabetes and Blood Sugar
Poorly controlled type 2 diabetes is a well-established risk factor for yeast infections. When blood sugar runs high, sugar levels in vaginal secretions also increase, giving Candida extra fuel to grow. The immune changes that come with diabetes, particularly reduced function of the white blood cells responsible for fighting fungal infections, compound the problem. Women with diabetes who keep their blood sugar well managed have a lower infection rate than those with uncontrolled levels.
Weakened Immune Defenses
Your immune system actively suppresses Candida growth through a specific pathway involving a type of immune cell that recruits infection-fighting white blood cells to the vaginal tissue. When this pathway is impaired, the body struggles to contain even normal amounts of yeast.
Several situations can weaken this defense. Corticosteroid medications, whether taken orally or inhaled, suppress immune activity broadly. Immunosuppressive drugs used after organ transplants or for autoimmune conditions have the same effect. HIV and other conditions that compromise immune function increase susceptibility to yeast infections significantly. Some people also carry inherited genetic variations that make their immune systems less effective at recognizing and responding to Candida, which helps explain why certain women experience chronic, recurrent infections despite no obvious trigger.
Douching and Vaginal Products
Douching disrupts the vaginal environment in several ways. Lab research shows that all common douching solutions are toxic to vaginal epithelial cells, the tissue that lines the vaginal wall. Baking soda-based douches specifically inhibit the growth of protective Lactobacillus species. Baking soda solutions also suppress the production of key immune-signaling molecules in vaginal tissue, weakening local defenses against infection.
When Lactobacillus bacteria are present alongside douching products, they partially protect vaginal cells from damage. But douching reduces those bacteria in the first place, creating a cycle: fewer protective bacteria means more cell damage, weaker immune signaling, and a more hospitable environment for Candida. The vagina is self-cleaning, and introducing products disrupts the very system designed to maintain balance.
Other Contributing Factors
Several everyday factors can shift conditions in favor of yeast growth. Tight, non-breathable clothing and synthetic underwear trap moisture and warmth near the vulva, which Candida prefers. Sitting in a wet swimsuit for hours creates similar conditions. Sexual activity doesn’t cause yeast infections directly (they’re not sexually transmitted), but it can introduce friction and minor tissue changes that alter the local environment.
Stress and sleep deprivation affect immune function broadly, and some women notice infections coinciding with high-stress periods. A diet extremely high in sugar has been proposed as a risk factor, though the evidence is less clear-cut than for the triggers above.
Why Some Infections Don’t Respond to Treatment
Most yeast infections are caused by Candida albicans, the species that responds well to standard antifungal treatments. But about 10 to 20% of infections involve other species, and these can be much harder to treat. One species has a resistance rate to the most commonly prescribed antifungal of nearly 80%, meaning standard treatment fails in the vast majority of cases. Another common non-albicans species shows resistance in about 16% of infections.
One useful clue: yeast infections typically don’t change vaginal pH. A healthy or mildly acidic reading around 4.0 points toward a yeast infection, while a pH above 4.5 suggests bacterial vaginosis, a different condition often confused with yeast infections. If over-the-counter treatments aren’t working after two or three attempts, the infection may involve a resistant species that requires a different antifungal approach, or it may not be a yeast infection at all.
Why Yeast Infections Keep Coming Back
Recurrent infections, defined as four or more per year, affect roughly 4 out of every 100 women of reproductive age globally. Recurrence often involves multiple overlapping causes rather than a single trigger. A woman on hormonal birth control who takes a course of antibiotics during a stressful month faces a compounding set of risk factors, each one nudging the vaginal environment further from balance.
In some cases, recurrence traces back to genetic differences in immune function. Variations in the genes that govern the body’s antifungal immune response can make certain women inherently more susceptible, regardless of lifestyle. For others, the issue is incomplete treatment: symptoms resolve before the fungal population is fully suppressed, and the remaining Candida rebounds. Understanding which factors apply to your situation is the key to breaking the cycle.