Yeast infections are caused by an overgrowth of a fungus called Candida, most commonly Candida albicans, that naturally lives on your skin and in your body. About 75% of women will experience at least one vaginal yeast infection in their lifetime, and 40% to 45% will have two or more. The fungus itself isn’t the problem. It’s part of a healthy microbiome. The infection starts when something disrupts the balance that normally keeps it in check.
How a Normal Fungus Becomes an Infection
Candida albicans lives quietly alongside bacteria and other microorganisms in the vagina, mouth, gut, and on skin. In its harmless state, it exists as round yeast cells. But when conditions shift, it can change shape, sprouting long, thread-like filaments called hyphae. This shape change is the starting point of infection. The filaments allow the fungus to physically attach to and penetrate the surface cells of your tissue, causing irritation and inflammation.
Once attached, the fungus can form a protective structure called a biofilm, a thin, layered colony that anchors itself to the tissue surface and becomes harder for your immune system to clear. Think of it like a film of algae on a rock in a stream. This is one reason yeast infections can be stubborn and why some people experience them repeatedly. The shift from harmless passenger to active infection doesn’t happen on its own. It requires a trigger, usually something that changes the local environment or weakens your body’s defenses.
Hormonal Changes and Estrogen
Estrogen plays a direct role in creating conditions where Candida thrives. Higher estrogen levels increase glycogen (a form of stored sugar) in vaginal tissue, which feeds both the beneficial bacteria and the yeast that live there. When the balance tips, that extra fuel gives Candida an advantage. Research shows that estrogen also helps the fungus evade your immune system by altering proteins on the fungal cell surface, making it harder for immune cells to recognize and destroy it.
This hormonal connection explains why yeast infections are more common during pregnancy, in the second half of the menstrual cycle (when estrogen peaks), and in people taking hormonal birth control with higher estrogen doses. It also explains why yeast infections are rare before puberty and after menopause, when estrogen levels are naturally low.
Antibiotics Are a Leading Trigger
Antibiotics are one of the most common causes of yeast infections. They kill the bacteria making you sick, but they also kill Lactobacillus and other beneficial bacteria that normally keep Candida in check in the vagina and on the skin. Without that bacterial competition, Candida can multiply rapidly and shift into its invasive form. Broad-spectrum antibiotics, the kind prescribed for a wide range of infections, carry the highest risk. The longer you take them, the more likely an overgrowth becomes.
Blood Sugar and Diabetes
Elevated blood sugar creates a favorable environment for yeast almost anywhere in the body. When blood glucose is high, excess sugar can appear in vaginal secretions and urine, essentially feeding the fungus directly. People with poorly controlled diabetes are significantly more likely to develop yeast infections, and those infections tend to be harder to resolve. The connection goes deeper than just sugar availability: high glucose also triggers Candida to produce surface proteins that resemble parts of the human immune system, which weakens the body’s ability to fight off the fungus.
This doesn’t mean you need a diabetes diagnosis to be affected. Any sustained period of high blood sugar, whether from diet, medication, or an undiagnosed condition, can increase your risk.
A Weakened Immune System
Your immune system is the main reason Candida stays in its harmless yeast form most of the time. When immunity is suppressed, the fungus has an easier path to overgrowth. Conditions that weaken the immune system, such as HIV, cancer treatment, organ transplant medications, and long-term corticosteroid use, all raise the risk of yeast infections. Even temporary immune dips from illness, stress, or sleep deprivation can be enough to tip the balance in some people.
Moisture, Heat, and Clothing
Candida thrives in warm, dark, moist environments. Sitting in a wet bathing suit, exercising in non-breathable fabrics, or wearing tight synthetic underwear all create exactly those conditions on your skin. The fungus doesn’t need much encouragement: trapped moisture raises the local temperature and reduces airflow, giving Candida the environment it needs to multiply.
Cotton underwear and loose-fitting clothing allow moisture to evaporate. Moisture-wicking fabrics designed for exercise help as well. Changing out of wet swimwear or sweaty workout clothes promptly is one of the simplest ways to reduce your risk.
Yeast Infections in Men
Men get yeast infections too, most commonly as balanitis, an infection of the head of the penis. The causes are largely the same: the Candida fungus is already present on the skin, and something allows it to overgrow. Risk factors for men include long-term antibiotic use, diabetes, a weakened immune system, being overweight, and poor hygiene. Uncircumcised men are more susceptible because the foreskin creates the warm, moist environment where Candida grows easily. Sexual contact with a partner who has a vaginal yeast infection also increases risk, though yeast infections are not considered a sexually transmitted infection in the traditional sense.
Why Some People Get Recurring Infections
For some people, yeast infections come back four or more times a year. This pattern, called recurrent vulvovaginal candidiasis, affects a meaningful number of the 40% to 45% of women who experience multiple episodes. Several factors can drive recurrence. Some people have subtle immune differences that make it harder to fully clear Candida. Others have infections caused by non-albicans species, particularly Candida glabrata, which is more resistant to standard antifungal treatments. Fluconazole resistance is a growing concern in vaginal yeast infections, meaning the most commonly prescribed treatment may not work for everyone.
Ongoing exposure to triggers also plays a role. If you’re taking frequent rounds of antibiotics, have persistently elevated blood sugar, or are on hormonal medications that keep estrogen high, the conditions for overgrowth keep resetting even after treatment clears the current infection. Identifying and addressing the underlying trigger is often more important than treating individual episodes.
Other Contributing Factors
Several additional factors can shift the vaginal environment enough to allow Candida overgrowth. Douching disrupts the natural bacterial balance. Scented soaps, sprays, and bubble baths can irritate tissue and alter local pH, creating an opening for the fungus. A diet very high in sugar and refined carbohydrates may contribute, though the evidence here is less direct than for diabetes-related blood sugar elevation. High stress levels can suppress immune function enough to matter over time. Even something as simple as staying in damp clothing after swimming on a hot day can be the final nudge the fungus needs.
In most cases, yeast infections result from a combination of factors rather than a single cause. Understanding which triggers apply to you makes prevention far more practical than treating infections after they appear.