Vaginal infections are typically caused by a disruption in the balance of microorganisms that naturally live in the vagina. A healthy vagina maintains an acidic environment (pH of 4.0 to 4.9) thanks to beneficial bacteria that convert glycogen into lactic acid. When that balance shifts, whether from antibiotics, sexual activity, hormonal changes, or lifestyle factors, harmful organisms can overgrow and cause infection.
The three most common types of vaginal infection each have different causes: bacterial vaginosis from an overgrowth of anaerobic bacteria, yeast infections from fungal overgrowth, and trichomoniasis from a sexually transmitted parasite. Understanding what triggers each one can help you recognize patterns and reduce your risk.
How the Vagina Normally Protects Itself
The vagina is not a sterile environment. It’s home to a community of microorganisms, dominated by beneficial bacteria that produce lactic acid. This lactic acid keeps the vaginal pH at or below 4.5, creating conditions that are inhospitable to most harmful bacteria and fungi. Across studies of healthy women, the median vaginal pH sits right around 4.5.
This protective system depends on estrogen. Estrogen stimulates the vaginal lining to produce glycogen, a type of stored sugar that beneficial bacteria feed on. When estrogen levels are stable and adequate, the vaginal lining stays thick and well-moisturized, and the acid-producing bacteria thrive. Anything that disrupts this chain, whether it kills off beneficial bacteria, changes hormone levels, or introduces new organisms, can open the door to infection.
Bacterial Vaginosis: An Overgrowth of Harmful Bacteria
Bacterial vaginosis (BV) is the most common vaginal infection. It happens when the balance between beneficial and harmful bacteria tips in the wrong direction. The protective, acid-producing bacteria lose ground, vaginal pH rises above 4.5, and anaerobic bacteria multiply. The hallmark symptoms are a thin, grayish-white discharge and a fishy odor.
Several things increase the risk of BV:
- New or multiple sexual partners. Sexual activity can introduce new bacteria into the vaginal environment, disrupting the existing microbial balance.
- Douching. Women who douche once a week are five times more likely to develop BV than women who don’t. Douching washes away beneficial bacteria and disrupts the natural acidity that keeps harmful organisms in check.
- Not using condoms. Semen is alkaline, which can temporarily raise vaginal pH and shift the bacterial balance over time with repeated exposure.
BV is not considered a sexually transmitted infection, but sexual activity is one of the strongest risk factors. It can also develop in women who aren’t sexually active, since anything that disrupts the vaginal microbiome can trigger it.
Yeast Infections: Fungal Overgrowth
Small amounts of Candida, the fungus responsible for yeast infections, normally live in the vagina without causing problems. Trouble starts when something allows the fungus to multiply unchecked. The result is thick, white, cottage cheese-like discharge along with itching, burning, and swelling.
Antibiotics are one of the most common triggers. Broad-spectrum antibiotics kill off large portions of the body’s bacterial communities, including the beneficial vaginal bacteria that keep Candida in check. Research shows that when antibiotics wipe out competing bacteria, they also release cellular fragments that actively stimulate the fungus to shift into a more aggressive, invasive form. Changes in gut chemistry caused by antibiotics, including shifts in bile acid levels, further promote fungal growth.
High blood sugar is another major driver. When blood glucose is elevated, excess sugar can be excreted in vaginal secretions and urine, essentially feeding yeast and bacteria in the area. Women with diabetes have a higher risk of yeast infections, particularly when their blood sugar isn’t well controlled.
Other factors that promote yeast overgrowth include pregnancy (when estrogen levels surge and alter the vaginal environment), hormonal contraceptives, and anything that creates a warm, moist environment around the vulva, such as sitting in wet clothing or wearing tight synthetic fabrics for extended periods.
Trichomoniasis: A Sexually Transmitted Parasite
Unlike BV and yeast infections, trichomoniasis is caused by an external organism: a single-celled parasite called Trichomonas vaginalis. It affects roughly 3.7 million people in the United States, with a prevalence of about 3.1% among women. The parasite spreads during sex without a condom, typically from a penis to a vagina, from a vagina to a penis, or from one vagina to another. It doesn’t commonly infect the hands, mouth, or anus.
Trichomoniasis often produces a frothy, yellow-green discharge with a strong odor, along with irritation and discomfort during urination or sex. However, many people with trich have no symptoms at all, which makes it easy to unknowingly pass along. Because it’s a parasitic infection rather than a microbial imbalance, the only way to get it is through sexual contact with someone who is already infected.
Hormonal Changes That Shift the Balance
Estrogen is the linchpin of vaginal health. It keeps the vaginal lining thick, elastic, and well-supplied with glycogen for beneficial bacteria to feed on. When estrogen levels drop or fluctuate significantly, the whole system can become unstable.
Menopause is the most dramatic example. As estrogen declines, the vaginal lining thins, moisture decreases, and the population of beneficial bacteria shrinks. The pH rises, creating an environment more hospitable to harmful organisms. This is why vaginal infections, particularly BV and yeast infections, can become more frequent during and after the menopausal transition. The vaginal microbiome is most stable during periods of peak estrogen, and least stable when estrogen is low.
Pregnancy pushes things in a different direction. Estrogen levels rise sharply, which increases glycogen production. While this feeds beneficial bacteria, the dramatic hormonal shift also changes the vaginal environment enough to promote yeast overgrowth in many women. The menstrual cycle itself creates smaller fluctuations: some women notice they’re more prone to infections at certain points in their cycle when hormone levels shift.
Lifestyle Factors That Increase Risk
Douching is one of the most well-documented risk factors for vaginal infection. It strips away beneficial bacteria, disrupts the natural acidic environment, and creates conditions for both BV and yeast infections to develop. If you already have an infection, douching can push bacteria upward into the uterus and fallopian tubes, potentially leading to pelvic inflammatory disease. Douching also increases susceptibility to sexually transmitted infections by removing bacteria that serve as a first line of defense.
What you wear matters more than you might expect. Synthetic fabrics trap heat and moisture against the vulva, creating exactly the warm, humid conditions that Candida and anaerobic bacteria thrive in. Research shows that nylon absorbs less sweat than cotton, keeping the groin area wetter and raising the risk of reproductive tract infections. Tight-fitting clothing compounds the problem by increasing friction and further limiting airflow. Cotton underwear and looser-fitting bottoms allow better ventilation.
Other lifestyle factors include staying in wet swimwear or workout clothes for extended periods, using scented soaps or sprays in the vaginal area (which can disrupt pH), and high stress levels, which can weaken the immune system’s ability to keep microbial populations in check.
When Multiple Factors Overlap
Vaginal infections rarely have a single cause. More often, it’s a combination of factors working together. A woman with diabetes who takes a course of antibiotics, for example, faces a compounded risk for yeast infection: the antibiotics kill off protective bacteria while elevated blood sugar feeds the fungus. Someone going through menopause who also douches has lost both hormonal protection and the bacterial defenses that douching washes away.
Recurrent infections, defined as three or more episodes in a year, often signal an underlying factor that hasn’t been addressed. This could be uncontrolled blood sugar, chronic stress, a hormonal shift, or an ongoing exposure like a sexual partner who carries Trichomonas without symptoms. Identifying and addressing these root causes is often more effective than treating each infection as a standalone event.