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 75% of women will experience at least one yeast infection in their lifetime, and roughly 138 million women worldwide deal with recurrent episodes every year. The overgrowth isn’t caused by “catching” something new. It’s a shift in the balance of organisms already living in your body.
How Yeast Lives in Your Body Without Causing Problems
Candida exists in a harmless form on your skin, in your gut, and in the vaginal canal. In its quiet state, it stays in a round, single-celled yeast shape and coexists peacefully with the other microorganisms in your body. Your vagina maintains an acidic environment, largely thanks to beneficial bacteria that produce lactic acid and hydrogen peroxide. These compounds keep Candida’s population low and prevent it from changing into a more aggressive form.
The key protective players are strains of Lactobacillus, particularly one called L. crispatus. These bacteria don’t just crowd out yeast by taking up space. They actively produce antimicrobial compounds that suppress Candida’s ability to grow and, crucially, prevent it from forming the structures it needs to cause damage. When this bacterial defense is intact, your body can tolerate even a moderate number of Candida cells without any symptoms at all.
What Triggers the Shift From Harmless to Harmful
The turning point is when Candida switches from its round yeast form into long, branching filaments called hyphae. This shape change is the core event behind a yeast infection. In its filament form, Candida can physically attach to the vaginal lining using specialized surface proteins, then invade the tissue through two methods: it can trick your cells into absorbing it, or it can actively push its way through. Once inside, it releases a toxin called candidalysin that punches holes in your cells, causing the inflammation, itching, and irritation you feel as symptoms.
Your body responds to this damage by sending immune cells to the area. In a best-case scenario, the immune response clears the fungus. But in many cases, the inflammation itself contributes to the burning and swelling that make yeast infections so uncomfortable. The infection is partly the fungus and partly your own immune system reacting to it.
Antibiotics Are a Leading Cause
Broad-spectrum antibiotics are one of the most common triggers for yeast infections because they don’t distinguish between harmful bacteria and beneficial ones. When you take antibiotics for a sinus infection, urinary tract infection, or anything else, those drugs also kill the Lactobacillus bacteria in your vagina. Without those protective bacteria producing acid and antifungal compounds, Candida faces no resistance and multiplies freely. This is why many women develop a yeast infection during or shortly after a course of antibiotics.
Hormones Play a Major Role
Estrogen directly influences the vaginal environment by stimulating cells in the vaginal lining to produce and store glycogen, a form of sugar. In normal circumstances, Lactobacillus bacteria break down glycogen into lactic acid, keeping the environment acidic. But higher estrogen levels mean more glycogen, which can also serve as fuel for yeast.
This is why yeast infections are more common during pregnancy, when estrogen levels surge. It also explains why some women notice infections around the luteal phase of their menstrual cycle, the stretch between ovulation and your period, when the vaginal lining thickens and glycogen deposits increase. Hormonal birth control containing estrogen can have a similar effect. Before puberty and after menopause, when estrogen is low, yeast infections are far less common.
Blood Sugar and Diabetes
Women with diabetes, particularly those with poorly controlled blood sugar, are significantly more prone to yeast infections. When blood glucose runs high, excess sugar shows up in vaginal secretions and urine, creating an environment rich in the nutrients Candida needs to thrive. The fungus essentially gets a steady food supply. Managing blood sugar levels reduces this risk, which is one reason recurrent yeast infections sometimes prompt doctors to screen for diabetes.
Clothing, Moisture, and Everyday Habits
Candida thrives in warm, moist environments. Tight-fitting clothing and synthetic underwear trap heat and moisture against the vulva, creating conditions that favor fungal growth. Cotton underwear wicks away sweat and allows airflow, which is why it’s consistently recommended. Synthetic fabrics with a small cotton crotch panel don’t offer the same protection because the surrounding material still restricts breathability. Panty liners worn daily can have a similar trapping effect.
Douching is another well-documented risk factor. It disrupts the natural balance of bacteria and acidity in the vagina, stripping away the protective organisms that keep Candida in check. A healthy vagina is self-cleaning, and introducing soaps, vinegar, or commercial douching products can directly trigger both yeast infections and bacterial vaginosis.
Weakened Immune System
Your immune system normally keeps Candida’s population controlled even when other risk factors are present. Conditions that suppress immune function, such as HIV, chemotherapy, or long-term use of corticosteroids, remove this safety net and allow yeast to proliferate more easily. Chronic stress and sleep deprivation can also subtly impair immune responses, though these are less dramatic triggers than the medical conditions above.
How to Tell It’s a Yeast Infection
The hallmark symptoms are itching, burning, and thick white discharge that’s often described as resembling cottage cheese. The discharge typically has no strong odor. You may also notice redness and swelling around the vulva, and pain during urination or sex.
These symptoms overlap with other conditions, so it’s worth knowing the differences. Bacterial vaginosis, the most common condition confused with yeast infections, produces a thin, grayish discharge that smells fishy. Yeast infection discharge is thicker, whiter, and odorless. Getting the wrong diagnosis matters because the treatments are completely different: antifungal medication treats yeast, while antibiotics treat bacterial vaginosis (and can actually cause a yeast infection in the process).
Why Some Women Get Them Repeatedly
Recurrent yeast infections, defined as three or more episodes in a single year, affect less than 5% of women. But the impact is significant: an estimated 372 million women globally will deal with recurrent infections over the course of their lives. In many cases, the Candida is never fully eliminated and re-emerges when conditions shift even slightly, such as a change in hormones, a round of antibiotics, or increased stress. Some women also have a genetic predisposition that makes their immune system less effective at recognizing and clearing the fungus.
Recurrent infections aren’t simply a hygiene issue, and they aren’t caused by sexual transmission in most cases. They reflect an ongoing vulnerability in the vaginal ecosystem that allows Candida to repeatedly shift into its invasive form. Treatment for recurrent cases typically involves longer courses of antifungal therapy and sometimes maintenance regimens designed to keep Candida suppressed over months.