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 three out of four women will experience at least one vaginal yeast infection in their lifetime. The overgrowth isn’t random. It’s triggered by specific shifts in vaginal chemistry, hormone levels, immune function, or everyday habits that tip the balance in the fungus’s favor.
How the Vagina Normally Keeps Yeast in Check
A healthy vagina isn’t sterile. It’s home to a community of microorganisms, and the dominant residents are bacteria from the Lactobacillus family. These bacteria act as a natural defense system against yeast in two key ways.
First, Lactobacillus bacteria produce lactic acid, which keeps vaginal pH in an acidic range of 3.8 to 4.5. That level of acidity makes it difficult for Candida to thrive. In lab studies, Lactobacillus strains acidified their environment to a pH between 3.7 and 4.7. Second, these bacteria produce hydrogen peroxide, a compound that directly inhibits fungal growth. Healthy vaginal environments are dominated by hydrogen peroxide-producing Lactobacillus strains.
When something disrupts this bacterial population, whether it’s antibiotics killing off Lactobacillus, hormonal shifts, or other factors, the pH rises, hydrogen peroxide levels drop, and Candida gets room to multiply. That’s when the itching, burning, and thick white discharge of a yeast infection begin.
Hormones and Estrogen’s Role
Estrogen has a direct relationship with yeast growth. It stimulates colonization of the vagina by Candida through two pathways: it promotes yeast growth directly, and it changes the vaginal lining in ways that make the environment more hospitable to fungus. Higher estrogen causes vaginal cells to store more glycogen (a type of sugar), which feeds Candida as those cells shed.
This explains why yeast infections cluster around specific life events. Pregnancy raises estrogen dramatically, making yeast infections far more common. Hormonal birth control, particularly high-estrogen formulations, can have a similar effect. Many women notice infections flaring in the second half of their menstrual cycle, when estrogen and progesterone peak. Hormone replacement therapy during menopause can also shift the balance toward yeast overgrowth.
Antibiotics Are a Major Trigger
Antibiotics are one of the most common and well-understood causes. They don’t target only the bacteria making you sick. They also wipe out the protective Lactobacillus in the vagina. Without those bacteria maintaining acidity and producing hydrogen peroxide, Candida faces little competition and can multiply rapidly. Broad-spectrum antibiotics, the kind prescribed for sinus infections, urinary tract infections, or respiratory illnesses, carry the highest risk because they kill the widest range of bacteria.
This is why yeast infections so often follow a course of antibiotics. The infection itself isn’t caused by the antibiotic. It’s caused by the sudden absence of the bacteria that were keeping yeast in check.
Blood Sugar and Diabetes
Chronically elevated blood sugar creates ideal conditions for yeast. When glucose levels in the blood are high, glucose concentrations in vaginal secretions rise too. That nutrient-rich environment directly promotes Candida proliferation. But the effect goes beyond just feeding the fungus. High glucose also enhances Candida’s ability to adhere to vaginal cells, the critical first step in establishing an active infection rather than just passive presence.
Women with poorly controlled type 1 or type 2 diabetes are significantly more prone to recurrent yeast infections. Even certain diabetes medications can contribute. A class of drugs called SGLT-2 inhibitors works by pushing excess glucose out through the urine, which increases glucose concentrations in the vulvovaginal area and creates a favorable environment for Candida growth. If you have diabetes and get frequent yeast infections, blood sugar management is often the most effective prevention strategy.
Immune System Suppression
Your immune system plays an active role in keeping vaginal Candida at harmless levels. Specific immune cells, particularly certain types of T cells, coordinate the antifungal response. When that immune surveillance weakens, Candida can shift from a quiet commensal organism into an aggressive infection.
Conditions that suppress immune function increase yeast infection risk substantially. HIV, chemotherapy, organ transplant medications, and long-term corticosteroid use all reduce the body’s ability to police fungal populations. Even significant stress or sleep deprivation can temporarily dampen immune responses enough to allow an overgrowth. This is also why some women find they get yeast infections when they’re run down or fighting another illness.
Clothing, Moisture, and Daily Habits
Candida thrives in warm, moist environments. Anything that traps heat and moisture against the vulva gives yeast an advantage. The CDC specifically recommends wearing cotton underwear, choosing breathable clothing that isn’t too tight, and keeping the area clean and dry to reduce risk.
In practical terms, this means changing out of wet swimsuits or sweaty workout clothes promptly, avoiding synthetic underwear that doesn’t breathe, and skipping tight leggings or jeans when possible if you’re prone to infections. Douching and scented vaginal products are also common triggers because they disrupt the Lactobacillus-dominated microbiome that keeps yeast suppressed.
Which Yeast Species Are Involved
Most vaginal yeast infections are caused by Candida albicans, but it’s not the only species involved. CDC surveillance data show that C. albicans accounts for about 37% of all Candida infections, followed by Candida glabrata at 30% and Candida parapsilosis at roughly 14%. The distinction matters because non-albicans species, particularly C. glabrata, can be more resistant to standard antifungal treatments. If you’ve treated a yeast infection with over-the-counter medication and it didn’t resolve, a different Candida species may be the cause.
Why Some Women Get Them Repeatedly
Recurrent yeast infections, defined as four or more episodes in a year, affect a smaller but significant group of women. Recurrence isn’t simply bad luck. It typically points to an ongoing underlying factor: persistent immune differences in how the body responds to Candida, uncontrolled blood sugar, continuous hormonal exposure, or a non-albicans species that isn’t fully cleared by standard treatment.
In women with recurrent infections, the immune response to Candida looks measurably different. The balance between protective and harmful immune signaling pathways shifts, allowing the fungus to reestablish itself after treatment. Some women also carry genetic variations in their immune receptors that make them inherently more susceptible to vaginal yeast overgrowth, regardless of their habits or other risk factors.
For recurrent infections, identifying and addressing the specific driver, whether that’s switching birth control, improving blood sugar control, or using a longer antifungal regimen, is more effective than treating each episode in isolation.