Yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, grows out of control. An estimated 75% of women will experience at least one yeast infection in their lifetime, and 40% to 45% will have two or more. The triggers range from hormonal shifts and antibiotics to everyday habits like clothing choices, and understanding them can help you reduce your risk.
How the Vaginal Microbiome Keeps Candida in Check
Your vagina is home to a community of bacteria dominated by Lactobacillus species. These bacteria produce lactic acid, hydrogen peroxide, and other antimicrobial substances that maintain a vaginal pH between 4 and 4.5. At that acidity level, Candida stays in its harmless yeast form and can’t easily transform into the elongated, thread-like shape (called hyphae) that allows it to invade tissue.
When something disrupts that bacterial population, pH rises, and Candida gets the opportunity to shift into its invasive form. Hyphae are highly adhesive, can penetrate the cells lining the vaginal wall, and release a toxin called candidalysin that directly damages tissue. Your immune system responds by flooding the area with inflammatory cells, but those cells often fail to clear the infection and end up causing collateral damage instead. That inflammatory loop is what produces the intense itching, burning, and swelling of a full-blown yeast infection.
Antibiotics Are the Most Common Trigger
Antibiotics kill bacteria, and they don’t distinguish between harmful bacteria and the protective Lactobacillus in your vagina. A course of broad-spectrum antibiotics for a sinus infection, urinary tract infection, or any other condition can dramatically reduce your Lactobacillus population, leaving Candida free to expand. This is one of the most well-established and frequent causes of yeast infections. The risk is highest with broad-spectrum antibiotics taken for longer courses, but even a short prescription can be enough in some women.
Hormonal Shifts and Estrogen
Elevated estrogen levels are strongly linked to yeast infections. Estrogen affects both the fungus itself and the vaginal lining, creating conditions that help Candida attach to cells and grow. This is why yeast infections are more common during pregnancy, when estrogen levels surge. Hormonal contraceptives, particularly higher-dose formulations, and hormone replacement therapy in postmenopausal women carry a similar increased risk.
The luteal phase of the menstrual cycle (the two weeks before your period) also brings a rise in progesterone and shifts in the vaginal environment, which is why some women notice yeast infections clustering around the same point in their cycle. Women who experience recurrent infections, defined as three or more episodes in a single year, sometimes find that the timing tracks closely with hormonal patterns. Recurrent infections affect fewer than 5% of women but can be difficult to manage.
High Blood Sugar Feeds the Fungus
Diabetes is a well-documented risk factor for yeast infections. When blood sugar is elevated, excess glucose shows up in vaginal secretions and urine, essentially feeding Candida a steady supply of its preferred fuel. Women with poorly controlled type 1 or type 2 diabetes are significantly more likely to develop recurrent infections. Even women without a diabetes diagnosis can be more vulnerable during periods of high sugar intake or insulin resistance, though the connection is strongest when blood sugar is consistently above normal ranges.
Weakened Immune Defenses
Your immune system normally keeps Candida colonization from progressing to a symptomatic infection. Anything that suppresses immune function shifts that balance. HIV/AIDS is the most dramatic example, but other conditions matter too: cancer, autoimmune diseases treated with immunosuppressive drugs, and chronic use of corticosteroids (including inhaled steroids for asthma) all reduce the body’s ability to control fungal growth. Chemotherapy has a similar effect by broadly suppressing immune cell production.
Douching and Fragrance Products
Douching disrupts the vaginal microbiome by physically washing away protective bacteria and altering pH. Research from the American Academy of Family Physicians found that douching at least once per month increased the risk of abnormal vaginal flora by 1.4 times, and douching within the past week more than doubled the risk. While that study focused on bacterial vaginosis specifically, the mechanism is the same one that opens the door to Candida overgrowth: depleted Lactobacillus and a less acidic environment.
Scented soaps, bubble baths, vaginal sprays, and scented tampons or pads can cause similar disruption. The vagina is self-cleaning, and introducing chemicals interferes with the microbial balance it maintains on its own.
Moisture, Clothing, and Everyday Habits
Candida thrives in warm, moist environments. Tight-fitting clothing and synthetic fabrics trap heat and moisture against the vulva, creating conditions the fungus loves. Cotton underwear wicks away sweat and moisture far more effectively than nylon or polyester. Even underwear with a cotton crotch panel doesn’t fully compensate for the synthetic fabric surrounding it, according to Cleveland Clinic experts.
A few practical habits that reduce moisture buildup:
- Change out of wet swimsuits and workout clothes as soon as possible rather than sitting in damp fabric.
- Skip underwear at night or wear loose pajamas to increase airflow, which is especially helpful during an active infection.
- Avoid panty liners for everyday use, as they decrease breathability and can trap irritation against the skin.
- Choose loose-fitting bottoms when you can, particularly in hot or humid weather.
Less Obvious Contributing Factors
Stress doesn’t cause yeast infections directly, but chronic stress suppresses immune function over time, which can tip the balance toward Candida overgrowth. Sleep deprivation has a similar indirect effect. A diet extremely high in refined sugar may contribute in women who are already prone to infections, though diet alone is rarely the sole cause.
Sexual activity doesn’t cause yeast infections (they’re not sexually transmitted), but friction and the introduction of new bacteria during intercourse can shift the vaginal environment enough to trigger an episode in some women. Spermicides and certain lubricants can also irritate vaginal tissue and alter pH.
Some women have a genetic predisposition that makes their immune system overreact to even small amounts of Candida, producing the intense inflammatory response responsible for symptoms. This helps explain why two women with identical habits and hormone levels can have very different experiences with yeast infections. For women dealing with recurrent episodes, this underlying immune tendency is often a bigger factor than any single external trigger.