Yeast infections happen when a fungus called Candida, which naturally lives in the vagina in small amounts, grows out of control. An estimated 75% of women will have at least one yeast infection in their lifetime, and 40% to 45% will have two or more. The triggers range from medications and hormonal shifts to everyday habits, and understanding them can help you reduce your risk.
What Keeps Yeast in Check
A healthy vagina has a surprisingly simple defense system. The vaginal microbiome is dominated by beneficial bacteria, particularly Lactobacillus species, which produce lactic acid and keep the vaginal pH between 4 and 4.5. At that acidic level, Candida stays in its harmless yeast form and can’t transition into the invasive, thread-like shape (called hyphae) that causes symptoms. Lactobacillus also produces hydrogen peroxide and other antimicrobial compounds that directly suppress Candida growth, and it strengthens the vaginal lining by boosting the proteins that hold cells tightly together.
When something disrupts this balance, whether it’s a medication, a hormonal change, or an immune system problem, Candida seizes the opportunity. It shifts into its invasive form, penetrates the vaginal lining, and releases a toxin that damages cells to extract nutrients like zinc and iron. That damage is what produces the itching, burning, and discharge most women recognize.
Antibiotics Are a Leading Trigger
Antibiotics kill bacteria. That’s their job. But they don’t distinguish between harmful bacteria and the protective Lactobacillus in your vagina. When those beneficial bacteria are wiped out, the acidic environment collapses and Candida can multiply unchecked.
A large postmarketing surveillance study in England found that women taking antibiotics had a dramatically higher risk of developing a yeast infection compared to women taking antidepressants (used as a control group). The risk peaked in the second week after starting treatment, reaching nearly 11 times higher, and remained significantly elevated through the third week. This pattern held across multiple antibiotic classes, including fluoroquinolones, cephalosporins, and macrolides. If you’ve ever gotten a yeast infection right after finishing a course of antibiotics, this is exactly why.
Hormonal Changes and Estrogen
Estrogen plays a direct role in yeast infection risk. Higher estrogen levels promote glycogen production in the vaginal lining, and glycogen is essentially a sugar-based fuel source that Candida thrives on. But estrogen does something even more concerning: it helps Candida evade the immune system by allowing the fungus to coat itself with a human protein that shields it from immune cells trying to destroy it.
This is why yeast infections spike during specific life stages and situations:
- Pregnancy: Estrogen levels rise significantly, especially in the second and third trimesters, making yeast infections much more common.
- High-estrogen oral contraceptives: Birth control pills with higher estrogen doses create a similar, though less dramatic, effect.
- Hormone replacement therapy: Women using estrogen-based HRT face an elevated risk for the same reasons.
Uncontrolled Blood Sugar and Diabetes
Women with diabetes, particularly those with poorly controlled blood sugar, are significantly more prone to yeast infections. The mechanism is straightforward: elevated blood glucose raises glycogen levels in vaginal tissue, which drops vaginal pH and creates an environment where Candida can colonize more aggressively. The sugar-rich environment also helps the fungus build protective biofilms, making infections harder to clear and more likely to recur.
This doesn’t apply only to diagnosed diabetics. Any sustained period of elevated blood sugar can shift the vaginal environment in Candida’s favor. Recurrent yeast infections in a woman without other obvious risk factors sometimes prompt a blood sugar screening for this reason.
A Weakened Immune System
Your immune system normally keeps Candida in its harmless state. When immune function is compromised, that surveillance breaks down. The CDC lists several conditions and medications that increase yeast infection risk:
- HIV/AIDS: Severely depleted immune cells make it difficult to control fungal growth anywhere in the body.
- Cancer and chemotherapy: Both the disease and its treatment suppress immune function.
- Corticosteroids: Oral or inhaled steroids (commonly used for asthma or autoimmune conditions) dampen the local and systemic immune response.
Clothing and Moisture
Candida thrives in warm, moist environments. Clothing choices that trap heat and sweat against the vulva create exactly those conditions. Synthetic fabrics like nylon and polyester don’t wick moisture the way natural fibers do, and tight-fitting clothing limits airflow.
Cotton underwear is the most commonly recommended alternative because it breathes and pulls moisture away from the skin. If you prefer synthetic underwear with a cotton crotch panel, know that the small panel doesn’t fully replicate the benefits of all-cotton fabric. Staying in wet swimsuits or sweaty workout clothes for extended periods creates a similar problem, giving yeast the damp conditions it needs to multiply.
Douching and Irritating Products
Douching disrupts the vaginal microbiome by physically washing out Lactobacillus and other protective bacteria. While bacterial counts begin recovering within a couple of hours, repeated douching keeps the microbiome in a state of flux. The epidemiologic evidence consistently shows that regular douching diminishes Lactobacillus dominance and increases the risk of reproductive tract infections, including yeast infections.
The vagina is self-cleaning. Introducing any product inside it, whether it’s a douche, scented soap, or feminine spray, risks altering the delicate chemical environment that keeps Candida suppressed. External washing with mild, unscented soap is all that’s needed.
Sexual Activity
Yeast infections are not classified as sexually transmitted infections, because you can get one without any sexual contact. But sex can play a role. You can acquire Candida from a sexual partner during vaginal, oral, or anal sex, and condoms or dental dams reduce that risk.
If your partner is male, his risk of developing symptoms from your infection is relatively low. About 15% of men develop an itchy rash on the penis after unprotected sex with a woman who has a yeast infection, with uncircumcised men and men with diabetes facing higher odds. If your partner is female, she may be at greater risk and should watch for symptoms. Sexual activity can also introduce bacteria and cause micro-friction that disrupts the vaginal environment, which is why some women notice infections correlating with periods of more frequent sex.
Why Some Women Get Recurring Infections
For the 40% to 45% of women who get two or more yeast infections, the triggers often overlap. A woman on oral contraceptives who takes an antibiotic has compounding risk factors. Someone with poorly controlled diabetes who wears synthetic underwear during workouts is stacking the odds further. Recurring infections aren’t typically a sign that something is seriously wrong, but they do suggest that one or more of these factors is consistently tipping the balance in Candida’s favor. Identifying which triggers apply to you is the most practical step toward breaking the cycle.