Vaginal yeast infections happen when a fungus called Candida, which normally lives in small amounts in the vagina, grows beyond what the body can keep in check. About 75% of women will experience at least one yeast infection in their lifetime, and roughly 138 million women worldwide deal with recurring episodes every year. The causes range from antibiotic use and hormonal shifts to underlying health conditions, but they all share a common thread: something disrupts the vagina’s natural defenses against fungal overgrowth.
How the Vagina Normally Keeps Yeast in Check
A healthy vagina is home to a community of bacteria, with Lactobacillus species playing the starring role. These bacteria produce lactic acid, which keeps the vaginal environment slightly acidic (typically a pH between 3.8 and 4.5). That acidity makes it difficult for Candida to multiply and take hold. Not all Lactobacillus strains are equally protective, though. Research shows that one species, Lactobacillus crispatus, produces significantly more lactic acid than another common strain, Lactobacillus iners, and inhibits Candida growth more effectively as a result.
When something kills off or reduces these protective bacteria, the vaginal pH shifts, and Candida gets an opening. The fungus transitions from a harmless presence into an active infection, triggering the itching, burning, and thick discharge that most women recognize immediately. Around 10 to 20% of healthy women carry Candida without any symptoms at all. The infection only develops when the balance tips.
Antibiotics Are the Most Common Trigger
Antibiotics are designed to kill bacteria, but they don’t distinguish between harmful bacteria and the beneficial Lactobacillus that protects the vaginal environment. This is why yeast infections so frequently follow a course of antibiotics, particularly broad-spectrum types like penicillin and amoxicillin. With the protective bacteria wiped out, Candida faces less competition and can multiply rapidly.
If you’ve ever noticed a yeast infection appearing a few days into a course of antibiotics for a sinus infection or strep throat, this is exactly what’s happening. The antibiotic solves one problem while inadvertently creating another. The risk is highest with broad-spectrum antibiotics taken orally, though it can occur with any type.
Hormonal Changes and Estrogen
Estrogen plays a significant and somewhat surprising role in yeast infections. Higher estrogen levels increase glycogen (a form of sugar) in vaginal tissue, which feeds Candida. Estrogen also suppresses parts of the local immune response, making it harder for the body to fight off fungal overgrowth. This dual effect is so reliable that researchers actually use estrogen to induce yeast infections in lab animals when studying treatments.
This explains why yeast infections cluster around certain life stages and situations:
- Pregnancy: Estrogen levels surge, especially in the second and third trimesters, making pregnant women particularly susceptible.
- Hormonal contraceptives: Birth control pills, patches, and rings that contain estrogen can raise infection risk, particularly higher-dose formulations.
- Hormone replacement therapy: Postmenopausal women taking estrogen-based HRT face increased risk compared to those who don’t.
- Menstrual cycle timing: Some women notice infections tend to appear in the luteal phase (the week or two before a period), when progesterone and estrogen fluctuate.
High Blood Sugar Feeds the Fungus
Women with diabetes, especially those with poorly controlled blood sugar, are at notably higher risk for yeast infections. The reason is straightforward: when blood glucose is elevated, excess sugar can appear in vaginal secretions and urine. Candida thrives on sugar, so a high-glucose environment is essentially a buffet for the fungus.
This doesn’t apply only to women with a diabetes diagnosis. Any sustained period of elevated blood sugar, whether from undiagnosed diabetes, prediabetes, or a diet very high in refined carbohydrates, can create conditions that favor yeast growth. Recurrent yeast infections in someone without other obvious risk factors are sometimes the first clue that points a doctor toward checking blood sugar levels.
A Weakened Immune System
Your immune system normally keeps Candida populations small even when conditions are otherwise favorable. When immunity is compromised, that surveillance breaks down. Several situations can weaken the immune response enough to allow yeast overgrowth:
- HIV/AIDS: Significantly increases susceptibility to all forms of Candida infection, not just vaginal.
- Corticosteroid medications: Oral steroids like prednisone suppress immune function broadly, including the body’s antifungal defenses.
- Chemotherapy: Cancer treatment suppresses the immune system as a side effect, raising infection risk across the board.
- Chronic stress and sleep deprivation: While less dramatic than the above, prolonged stress elevates cortisol, which can dampen immune function over time.
Not All Yeast Infections Are the Same Species
Most vaginal yeast infections are caused by Candida albicans, but a meaningful minority involve other species. Candida glabrata is the most common non-albicans culprit, and it matters because it behaves differently. C. glabrata has inherently low susceptibility to the standard antifungal treatments (the azole class, including the fluconazole pill that many women are familiar with). This means infections caused by this species often don’t respond to first-line treatment and require alternative approaches.
If you’ve treated what seems like a straightforward yeast infection with an over-the-counter product and it hasn’t resolved, a non-albicans species could be the reason. It could also mean the symptoms aren’t from yeast at all. Getting a proper culture done, rather than relying on self-diagnosis, becomes especially important with persistent or unusual symptoms.
Why Some Women Get Recurring Infections
Recurrent yeast infections, generally defined as three or more episodes within a single year, affect fewer than 5% of women but carry a significant quality-of-life burden. Globally, about 138 million women deal with recurrent infections annually, with the highest rates in the 25 to 34 age group, where prevalence reaches about 9%. Over a lifetime, an estimated 372 million women experience recurrent episodes.
Recurrence often involves a combination of the factors above rather than a single cause. A woman on hormonal birth control who periodically takes antibiotics and has a genetic tendency toward a less protective vaginal microbiome may find herself in a repeating cycle. In some cases, Candida forms biofilms on vaginal tissue, which are thin, sticky colonies that resist both immune defenses and antifungal treatments, allowing the fungus to persist at low levels and flare up repeatedly.
For women caught in a recurrence pattern, identifying and addressing the underlying contributors matters more than simply treating each episode as it comes. That might mean evaluating hormonal contraception choices, screening for diabetes, or using longer-duration antifungal regimens designed to fully suppress the fungus rather than just knock it back temporarily.
Common Factors That Don’t Cause Infections but May Contribute
Several everyday habits don’t directly cause yeast infections but can tip the scales in an already-vulnerable environment. Wearing tight, non-breathable underwear or sitting in a wet swimsuit creates a warm, moist environment where yeast thrives more easily. Douching disrupts the vaginal microbiome in much the same way antibiotics do, killing off protective bacteria. Scented soaps, bubble baths, and vaginal deodorants can irritate tissue and alter pH.
These factors alone rarely cause an infection in someone with a robust vaginal microbiome and normal immune function. But combined with a course of antibiotics or a hormonal shift, they can be enough to push things over the edge. Switching to cotton underwear, avoiding internal cleansing products, and changing out of damp clothing promptly are simple measures that reduce, though don’t eliminate, risk.