Vaginal yeast infections are caused by an overgrowth of a fungus called Candida that naturally lives in your body. Under normal conditions, other bacteria keep this fungus in check. But when something disrupts that balance, Candida multiplies out of control and triggers the itching, burning, and discharge that roughly 75% of women will experience at least once in their lives. About 40% to 45% will deal with two or more episodes.
The triggers behind that disruption range from hormonal shifts and medications to everyday habits you might not think twice about. Understanding what tips the balance can help you reduce your risk.
How Your Vaginal Ecosystem Normally Keeps Yeast in Check
Your vagina maintains its own protective environment through a community of beneficial bacteria, primarily Lactobacillus species. These bacteria produce lactic acid, which keeps your vaginal pH slightly acidic, typically between 3.8 and 4.5. That acidity creates a hostile environment for Candida and other harmful organisms, limiting their ability to grow and colonize the vaginal walls.
Candida is always present in small amounts. It lives on your skin, in your gut, and in your vagina without causing problems, as long as Lactobacillus and other bacteria hold it in check. A yeast infection isn’t about “catching” something new. It’s about losing the conditions that kept an existing resident under control.
Antibiotics Are the Most Common Trigger
Antibiotics are one of the most frequent causes of yeast infections because they don’t distinguish between harmful bacteria and the beneficial Lactobacillus in your vagina. When you take antibiotics for a sinus infection, urinary tract infection, or any other illness, the medication can wipe out the protective bacteria that maintain your vaginal acidity. With that defense weakened, Candida has space and opportunity to multiply rapidly.
This is why yeast infections so often follow a course of antibiotics, sometimes appearing within days of finishing the prescription. Broad-spectrum antibiotics, which target a wide range of bacteria, carry the highest risk.
Hormones and Estrogen’s Role
Estrogen has a direct relationship with yeast growth. It stimulates the cells lining your vaginal walls to produce glycogen, a form of stored sugar. Candida albicans, the species responsible for most vaginal yeast infections, feeds on glycogen. Research published in the Australian and New Zealand Journal of Obstetrics and Gynaecology confirmed that all 34 tested strains of C. albicans could break down and use glycogen as fuel, while 26 other Candida species could not. This is one reason C. albicans dominates vaginal yeast infections specifically.
Estrogen also changes the structure of vaginal tissue, making it easier for yeast to adhere to the vaginal walls. This explains why yeast infections cluster around certain life stages and events:
- Pregnancy: Estrogen levels rise dramatically, increasing glycogen production and making yeast infections significantly more common, especially in the second and third trimesters.
- Hormonal contraceptives: Birth control pills, patches, and rings that contain estrogen can raise your baseline risk by keeping estrogen levels elevated.
- Menstrual cycle timing: Many women notice infections appearing in the days before their period, when estrogen and progesterone shift rapidly.
- Hormone replacement therapy: Estrogen-based HRT during menopause can reintroduce the same glycogen-rich conditions that favor Candida.
Before puberty and after menopause (without HRT), yeast infections are relatively uncommon, precisely because estrogen levels are low and the vaginal environment produces less glycogen.
High Blood Sugar Feeds Yeast Directly
Women with diabetes face a higher risk of yeast infections, particularly when blood sugar is poorly controlled. The CDC notes that elevated blood glucose leads to excess sugar being released through urine and vaginal secretions, creating a nutrient-rich environment where yeast thrives. The effect is essentially the same as the estrogen-glycogen pathway: more sugar available means more fuel for Candida.
This doesn’t only apply to diagnosed diabetes. Periods of high sugar intake, insulin resistance, or prediabetes can also shift your vaginal chemistry enough to encourage yeast overgrowth. If you’re getting recurrent yeast infections without an obvious cause, it may be worth having your blood sugar checked.
A Weakened Immune System
Your immune system plays a constant, quiet role in keeping Candida populations small. When immune function is compromised, that surveillance weakens. Conditions and treatments that suppress immunity include HIV, chemotherapy, organ transplant medications, and long-term corticosteroid use (like prednisone). Even high stress and sleep deprivation can temporarily dampen immune responses enough to give yeast an opening.
Women who are otherwise healthy but dealing with a period of intense stress or illness sometimes develop a yeast infection seemingly out of nowhere. The immune dip doesn’t have to be severe to matter.
Douching and Scented Products
Douching is one of the clearest self-inflicted risk factors. According to the Office on Women’s Health, douching removes the normal bacteria that protect your vagina from infection and disrupts the natural acidity that keeps harmful organisms in check. The result can be an overgrowth of either harmful bacteria or yeast.
Scented tampons, pads, sprays, and powders pose a similar risk. These products can irritate vaginal tissue and alter its chemistry, creating conditions that favor Candida. Your vagina is self-cleaning. Products marketed for vaginal freshness work against the system your body already has in place.
Clothing and Moisture
Candida thrives in warm, moist environments. Anything that traps heat and moisture against the vulva for extended periods can encourage fungal growth. A study in the Brazilian Journal of Sexually Transmitted Diseases found that women diagnosed with vulvovaginitis were significantly more likely to wear synthetic fabric underwear compared to those without infections. Among women with infections, 10.6% regularly wore synthetic underwear, compared to zero in the control group.
Tight-fitting pants, leggings, and workout clothes that aren’t changed promptly after exercise create the same kind of occlusive, humid environment. Wet swimsuits worn for hours have a similar effect. Cotton underwear and changing out of sweaty or wet clothing relatively quickly reduces the moisture that Candida needs to flourish.
Other Contributing Factors
Several additional factors can shift the balance toward yeast overgrowth:
- Sexual activity: Yeast infections aren’t sexually transmitted, but intercourse can introduce new bacteria and change vaginal pH, occasionally triggering an episode. Saliva, lubricants, and spermicides can all alter vaginal chemistry.
- Spermicides and certain lubricants: Products containing nonoxynol-9 can irritate vaginal tissue and disrupt bacterial balance.
- Sitting in wet or damp conditions: Hot tubs, prolonged baths, and staying in wet swimwear all create favorable conditions for yeast.
Why Some Women Get Recurrent Infections
Recurrent yeast infections, generally defined as four or more episodes in a single year, affect a meaningful percentage of women. For some, a clear trigger exists: ongoing antibiotic use, uncontrolled diabetes, or chronic immunosuppression. But for many, recurrent infections happen without an obvious explanation.
In some cases, the issue is a Candida strain that’s particularly persistent or resistant to standard antifungal treatments. In others, subtle immune differences in the vaginal lining may make certain women less effective at suppressing Candida regrowth even when their overall immune system is healthy. Genetic variation in how your body’s local immune cells respond to yeast appears to play a role, though this is still being studied. If you’re dealing with frequent infections, identifying and addressing any modifiable risk factors, like blood sugar, clothing choices, or unnecessary antibiotic use, is the most practical starting point.