Yeast infections happen when a fungus called Candida, which normally lives in small amounts on your skin and inside your body, grows out of control. About 75% of women will get at least one vaginal yeast infection in their lifetime, and the triggers range from medications and hormonal shifts to everyday habits like the underwear you choose. Understanding what tips the balance can help you recognize patterns and reduce your risk.
How Yeast Overgrowth Actually Starts
Candida isn’t an invader. It already lives on your mucosal surfaces, including your gut, mouth, and vagina, without causing problems. Your body can tolerate a low, moderate, or even high number of Candida cells as long as certain checks stay in place. The trouble starts when something disrupts that balance and the fungus shifts from a harmless passenger to an aggressive one.
The key shift happens at a cellular level: Candida changes shape from a round yeast form into elongated filaments that can burrow into tissue. Once those filaments dig in, the fungus releases a toxin that punches holes in your cells and triggers inflammation. That’s what causes the redness, swelling, and itching you feel. The question, then, is what knocks down the defenses that normally keep Candida in its harmless form.
The Role of Protective Bacteria
Your vagina maintains its own ecosystem, dominated by beneficial bacteria called lactobacilli. These bacteria produce lactic acid that keeps vaginal pH between 4 and 4.5, an acidic range where Candida’s ability to form those tissue-invading filaments is limited. Lactobacilli also compete with Candida for nutrients and attachment sites on vaginal cells, produce hydrogen peroxide and antimicrobial compounds, and strengthen the lining of the vaginal wall by boosting the proteins that hold cells tightly together.
Anything that weakens or depletes this bacterial population opens the door for Candida to take over. That’s the common thread connecting most of the causes below.
Antibiotics
Antibiotics are one of the most well-documented triggers. Broad-spectrum antibiotics don’t just kill the bacteria causing your sinus infection or urinary tract infection. They also wipe out the protective lactobacilli in your vagina, leaving Candida with less competition. A study of over 1,500 women found that those who used antibiotics in the month before their visit were 75% more likely to have a vaginal yeast infection than those who didn’t. The risk climbed with longer courses of antibiotics, and women with a history of recurrent yeast infections were especially vulnerable.
This doesn’t mean you should skip a prescribed antibiotic. But if you’ve noticed a pattern of yeast infections following antibiotic courses, that connection is real and worth discussing with your provider.
Hormonal Changes
Estrogen plays a direct role in the vaginal environment. Higher estrogen levels increase the amount of glycogen (a sugar stored in cells) in your vaginal lining. While some glycogen feeds the beneficial lactobacilli, excess amounts also give Candida more fuel to grow. This is why yeast infections tend to cluster around specific life stages and hormonal events.
Pregnancy is a common trigger because estrogen levels surge throughout all three trimesters. Hormonal birth control, particularly higher-dose estrogen pills, can have a similar effect. Some women also notice yeast infections flare in the second half of their menstrual cycle, when estrogen and progesterone are both elevated. On the flip side, menopause brings a drop in estrogen that reduces glycogen, which can cause lactobacilli to decline and leave you more prone to infections for a different reason: without enough good bacteria, the vaginal pH rises and the whole ecosystem shifts.
Women on vaginal estrogen therapy for menopausal symptoms sometimes find themselves dealing with yeast infections again, because the supplemental estrogen recreates the high-glycogen environment Candida thrives in.
High Blood Sugar and Diabetes
When your blood sugar runs high, your body releases excess sugar through mucous membranes and urine. That sugar-rich environment in and around the vagina is essentially a feast for Candida. The CDC specifically notes that women with diabetes are at higher risk for vaginal yeast infections, especially when blood sugar is poorly controlled.
This applies whether you have type 1 or type 2 diabetes. Recurrent yeast infections in someone who hasn’t been diagnosed with diabetes can sometimes be an early clue that blood sugar levels need checking. If you’re getting yeast infections frequently and can’t identify another cause, it’s worth having your blood sugar tested.
Weakened Immune System
Your immune system normally keeps Candida populations in check. When that surveillance weakens, the fungus can grow unchecked. Several situations can suppress your immune response enough to trigger yeast overgrowth:
- HIV/AIDS reduces the white blood cells that patrol mucosal surfaces, making oral and vaginal yeast infections common and sometimes severe.
- Chemotherapy suppresses the immune system broadly, increasing vulnerability to all types of Candida infections.
- Corticosteroids dampen inflammation but also suppress immune defenses. Inhaled steroids for asthma are a well-known cause of oral thrush (a yeast infection of the mouth), while oral or systemic steroids can increase risk elsewhere in the body.
- Cancer and organ transplant medications deliberately lower immune function, creating an opening for Candida.
Clothing and Moisture
Candida thrives in warm, moist environments. Anything that traps heat and moisture against the vulva creates conditions that favor yeast growth. Synthetic fabric underwear, tight leggings, wet swimsuits, and even panty liners can reduce breathability and hold moisture against the skin.
Cotton underwear wicks away excess sweat and moisture that yeast and bacteria feed on. If your synthetic underwear has a cotton panel in the crotch, that small strip doesn’t fully protect you from the surrounding non-breathable fabric. Changing out of workout clothes or wet bathing suits promptly, and choosing looser-fitting bottoms when possible, reduces the warm, damp conditions Candida prefers.
Douching and Hygiene Products
Douching disrupts the vaginal ecosystem by washing away the hydrogen peroxide and lactobacilli that protect against overgrowth. Research shows that douching at least once per month raises the risk of abnormal vaginal flora by 1.4 times, and douching within the past week more than doubles it. While these studies focused primarily on bacterial vaginosis, the mechanism is the same: stripping away protective bacteria creates an opening for any opportunistic organism, including Candida.
Scented soaps, bubble baths, vaginal sprays, and scented tampons can similarly irritate vaginal tissue and alter pH. The vagina is self-cleaning, and plain warm water on the external area is all that’s needed for routine hygiene.
Sexual Activity
Yeast infections are not sexually transmitted infections, but sexual activity can still set the stage for one. Semen is alkaline, with a pH around 7 to 8, which temporarily raises vaginal pH and can disrupt the acidic environment that keeps Candida in check. Lubricants can also alter vaginal pH. Friction during intercourse may cause micro-irritation to vaginal tissue, which can make it easier for Candida to gain a foothold.
Using condoms reduces direct contact with semen and helps maintain a more stable vaginal pH. Choosing pH-balanced or water-based lubricants can also help if you’ve noticed a connection between sex and yeast infections.
Less Common Yeast Species
Most vaginal yeast infections are caused by Candida albicans, but other species can be responsible, and they sometimes behave differently. Candida glabrata is the second most common culprit and is more likely to resist standard antifungal treatments. Around 5% of invasive Candida glabrata cases are resistant to one or more antifungal medications, and resistance to first-line treatments is increasing. If you’re treating a yeast infection with over-the-counter antifungals and it isn’t improving, a non-albicans species could be the reason, and lab testing can identify which one you’re dealing with.
Multiple Triggers Often Overlap
In practice, yeast infections rarely come down to a single cause. A woman on antibiotics who also wears tight synthetic clothing and has slightly elevated blood sugar is stacking risk factors. Recognizing which ones apply to you is the first step toward breaking a pattern of recurrent infections. Keeping track of when infections occur, what medications you were taking, where you were in your menstrual cycle, and what you were wearing can reveal connections that aren’t obvious at first.