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 in their lifetime. The trigger isn’t a single cause but rather anything that disrupts the delicate balance between vaginal bacteria, yeast, and the immune system.
What Keeps Yeast Under Control
A healthy vagina maintains a slightly acidic environment, with a pH between 4.0 and 4.5 during reproductive years. This acidity is produced by beneficial bacteria called lactobacilli, which create lactic acid as a byproduct. Lactobacilli also physically attach to the vaginal lining and compete with Candida for space, forming a biological barrier that keeps yeast from gaining a foothold.
When this system is working, Candida stays in its harmless yeast form. But when something disturbs the balance, the fungus can shift into a more aggressive form that burrows into vaginal tissue and triggers inflammation. That shift is what produces the itching, burning, and thick white discharge most people associate with a yeast infection.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics are the single most recognized cause of yeast infections. These medications kill the bacteria causing your illness, but they also wipe out the protective lactobacilli in the vagina. Without those bacteria keeping Candida in check, yeast multiplies rapidly. Not all antibiotics carry this risk equally. Narrow-spectrum antibiotics, which target a specific type of bacteria, are less likely to cause problems. Broad-spectrum varieties, prescribed for things like sinus infections or urinary tract infections, are the ones most often linked to yeast overgrowth.
If you’ve noticed a pattern of getting yeast infections after a course of antibiotics, you’re not imagining it. It’s one of the most predictable triggers and worth mentioning to your prescriber so they can choose the narrowest effective antibiotic when possible.
Hormones and Blood Sugar
Estrogen plays a direct role in how hospitable the vagina is to yeast. Higher estrogen levels increase the amount of glycogen (a sugar stored in tissue) in the vaginal lining. While glycogen normally feeds the beneficial lactobacilli, an excess can also fuel Candida growth. This is why yeast infections are more common during pregnancy, in the second half of the menstrual cycle, and sometimes with hormonal birth control.
After menopause, the picture flips. Estrogen drops, the vaginal lining thins, lactobacilli populations decline, and pH rises above 4.5 into a more alkaline range. This creates a different set of infection risks, though yeast infections become less common at this stage.
Blood sugar matters too. The CDC notes that women with diabetes face a higher risk of yeast infections, particularly when blood sugar is poorly controlled. When glucose levels run high, excess sugar can end up in vaginal secretions and urine, essentially feeding Candida a steady supply of its preferred fuel. Keeping blood sugar well managed significantly reduces this risk.
Hygiene Products That Backfire
Scented tampons, pads, vaginal washes, and douches can all shift vaginal pH away from its protective acidic range. The chemicals and fragrances in these products disrupt lactobacilli populations, and once pH rises, the balance between bacteria and yeast tips in Candida’s favor. The vagina is self-cleaning, and introducing anything scented or chemically formulated into it tends to create the very problems these products claim to prevent.
Douching is particularly disruptive. It flushes out both beneficial and harmful organisms, leaving a blank slate that Candida often colonizes faster than lactobacilli can recover. Plain water for external washing is sufficient. Internal cleansing products do more harm than good.
Clothing, Moisture, and Everyday Habits
Yeast thrives in warm, moist environments. Tight clothing made from synthetic fabrics like nylon or polyester traps heat and moisture against the vulva, creating ideal conditions for Candida to multiply. Cotton underwear is breathable and wicks moisture away, which is why it’s consistently recommended as the better choice for everyday wear. Sitting in a wet swimsuit or sweaty workout clothes for extended periods has the same effect as synthetic fabrics: it keeps the area damp and warm.
Changing out of wet or sweaty clothing promptly, choosing cotton or cotton-lined underwear, and avoiding excessively tight pants or leggings when possible all help keep moisture levels in a range that discourages yeast overgrowth.
Weakened Immunity
Your immune system is the other major force keeping Candida in its harmless state. Anything that suppresses immune function, including chronic stress, sleep deprivation, HIV, or medications like corticosteroids and chemotherapy, can allow yeast to transition from a quiet passenger to an active infection. For people with intact immune systems, a yeast infection is an uncomfortable nuisance. For those with compromised immunity, infections can be more severe and harder to resolve.
Is It Sexually Transmitted?
Yeast infections are not typically considered sexually transmitted. Most occur without any sexual contact, and routine treatment of sexual partners is not recommended. That said, a small percentage of male partners may develop redness and irritation on the penis after contact with someone who has an active infection. In those cases, a topical antifungal can relieve their symptoms. But the infection itself almost always originates from the person’s own Candida rather than being passed between partners.
When Infections Keep Coming Back
Some people experience four or more yeast infections in a single year, which is classified as recurrent. Recurrent infections can be frustrating because the standard short-course treatments that clear a one-time infection often aren’t enough. The underlying triggers, whether hormonal, immune-related, or tied to an ongoing medication, may need to be addressed directly. Recurrent cases sometimes involve Candida species other than the most common one, which can be resistant to standard over-the-counter treatments. A culture or lab test, rather than self-diagnosis, becomes especially important when infections keep returning.
Keeping a log of when infections occur relative to your menstrual cycle, antibiotic use, or other medications can help identify patterns that make recurrence easier to prevent.