Vaginal yeast infections are caused by an overgrowth of Candida, a type of fungus that naturally lives in the vagina in small amounts. Up to 75% of women will experience at least one yeast infection in their lifetime. The overgrowth happens when something disrupts the balance of bacteria and yeast in the vagina, giving Candida the opportunity to multiply unchecked.
How the Vaginal Microbiome Keeps Yeast in Check
Your vagina maintains its own ecosystem. Protective bacteria, particularly a species called Lactobacillus, produce lactic acid that keeps the environment acidic (below pH 4.5). That acidity suppresses Candida growth and prevents it from gaining a foothold. When this bacterial population is healthy and dominant, yeast stays at low, harmless levels.
Not all protective bacteria are equally effective. Research shows that women whose vaginal flora is dominated by one particular strain, Lactobacillus crispatus, have stronger defenses against Candida colonization. Women with a different dominant strain, Lactobacillus iners, are nearly three times more likely to harbor Candida. This helps explain why some women seem more prone to yeast infections than others, even without an obvious trigger.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics are one of the most well-known causes. These medications kill a wide range of bacteria, including the protective Lactobacillus in the vagina. With fewer bacteria producing lactic acid, the vaginal pH shifts, and Candida can multiply rapidly. This is why yeast infections commonly show up during or shortly after a course of antibiotics for an unrelated illness like a sinus infection or urinary tract infection.
Hormones and Estrogen Levels
Estrogen plays a direct role in yeast infection risk. High estrogen levels increase the amount of glycogen (a type of sugar) stored in vaginal cells, which feeds Candida and encourages overgrowth. This is why yeast infections are more common during pregnancy, when estrogen levels surge. Women taking birth control pills with higher estrogen doses face a similar increase in risk.
Menopause creates a different pattern. Estrogen drops significantly, which thins the vaginal walls and changes the microbial balance. While lower estrogen alone doesn’t directly feed yeast, women who start hormone replacement therapy or use vaginal estrogen to manage menopause symptoms often notice more frequent yeast infections as a side effect. The reintroduction of estrogen recreates the conditions that favor Candida growth.
Blood Sugar and Diabetes
Women with diabetes are at higher risk for yeast infections, especially when blood sugar is poorly controlled. The connection is straightforward: when blood sugar runs high, excess glucose can be released through vaginal secretions and urine. That extra sugar acts as fuel for Candida, allowing it to multiply faster than the body can keep it in check. Bringing blood sugar under tighter control typically reduces the frequency of infections.
Weakened Immune System
Your immune system normally helps keep Candida populations suppressed. Conditions that weaken immune function, including HIV/AIDS, cancer, and cancer treatment like chemotherapy, increase susceptibility to yeast infections. Long-term use of corticosteroids (including inhaled steroids for asthma) can have a similar effect by dampening the body’s immune response.
Interestingly, vaginal yeast infections follow different immune rules than yeast infections in the mouth or throat. Defects in the adaptive immune system (the part that “learns” to fight specific infections) don’t increase vaginal yeast infection risk. Instead, the vaginal immune response relies more on innate defenses. Certain genetic variations in these innate immune pathways, particularly in receptors that detect fungal cells, are associated with recurrent infections. This means some women are genetically more susceptible regardless of their lifestyle.
Douching and Hygiene Products
Douching disrupts the vaginal environment by washing away protective bacteria and altering pH. Women who douche at least once a month have a 1.4 times higher risk of vaginal flora disruption, and the risk jumps to 2.1 times higher when douching occurs within the week. While most of this research focuses on bacterial vaginosis rather than yeast specifically, the mechanism is the same: stripping away Lactobacillus removes the barrier that keeps opportunistic organisms, including Candida, under control.
Scented soaps, bubble baths, and feminine sprays can cause similar disruption. The vagina is self-cleaning, and introducing products with fragrances or harsh chemicals interferes with the microbial balance it maintains on its own.
Other Contributing Factors
Several everyday factors can tip the balance toward yeast overgrowth:
- Tight or non-breathable clothing: Synthetic underwear, wet swimsuits, and tight pants create a warm, moist environment where yeast thrives.
- Menstrual cycle timing: Hormonal shifts around your period can temporarily alter the vaginal environment, making infections more likely at certain points in your cycle.
- Sexual activity: While yeast infections aren’t sexually transmitted, sexual activity can introduce bacteria and change vaginal pH, which may trigger overgrowth in some women.
When Infections Keep Coming Back
About 5 to 8% of women experience recurrent yeast infections, defined as four or more episodes in a single year. Recurrent infections sometimes involve non-albicans species of Candida, particularly Candida glabrata, which is more resistant to standard over-the-counter treatments. These species don’t always respond to fluconazole, the most commonly used antifungal, which is why infections that don’t clear up with typical treatment may need a lab culture to identify the specific yeast involved.
For women with recurrent infections and no obvious trigger like antibiotic use or uncontrolled diabetes, genetic factors in the innate immune system are increasingly recognized as a driver. Variations in specific immune receptors that detect fungal cells have been linked to higher rates of recurrent infections. This means the cause isn’t always something you’re doing wrong. For some women, it’s a matter of biology.
What Yeast Infection Symptoms Look Like
The hallmark symptoms are intense vulvar itching, along with a thick, white, clumpy discharge often described as resembling cottage cheese. You may also notice redness, swelling, or soreness around the vulva, and burning during urination or sex. Small cracks or raw patches on the skin around the vaginal opening can also develop in more severe cases. Unlike bacterial infections, yeast infections typically don’t produce a strong or fishy odor, which can help distinguish the two.