Bacterial vaginosis (BV) develops when the natural balance of bacteria inside the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment, keeping it slightly acidic and inhospitable to harmful organisms. When something disrupts that balance, those protective bacteria decline and a mix of anaerobic bacteria rapidly overgrow to take their place. An estimated 23 to 29 percent of women of reproductive age worldwide have BV at any given time, making it the most common vaginal condition in that age group.
BV is not caused by a single germ you “catch” the way you catch a cold. It’s the result of an ecosystem collapse inside the vagina, and multiple factors can trigger it.
What Happens Inside the Vagina
A healthy vagina is home to large colonies of lactobacilli. These bacteria produce lactic acid, which keeps vaginal pH below 4.5. That acidic environment acts as a natural defense, suppressing the growth of potentially harmful organisms that are always present in small numbers.
When lactobacilli populations drop, vaginal pH rises and becomes more alkaline. That shift opens the door for anaerobic bacteria to multiply rapidly. The organisms involved include several species that are normally present in tiny amounts but can cause problems in large numbers. The result is the thin, grayish-white discharge and fishy odor that are hallmarks of BV. This isn’t an infection from the outside so much as an internal imbalance that tips out of control.
Sexual Activity and BV
BV is not classified as a sexually transmitted infection, but sexual activity is one of the strongest risk factors. Sex appears to change the bacterial environment inside the vagina, making overgrowth more likely. New sexual partners, multiple partners, and unprotected sex all increase risk. The exact mechanism isn’t fully understood, but semen is alkaline, and the introduction of a partner’s bacteria can disrupt the vaginal ecosystem.
Women who have sex with women can also pass the bacteria associated with BV between partners, particularly through shared sex toys or oral sex. Covering toys with a condom (and replacing the condom between partners) and using dental dams during oral sex can reduce this risk. Men don’t develop BV themselves, but they can carry the bacteria on their body and transfer it to a new partner.
It’s worth noting that women who have never had sex can still develop BV. Sexual activity raises the odds, but it isn’t required.
Douching and Hygiene Products
Douching is one of the most well-documented triggers. Rinsing the inside of the vagina with water, vinegar, or commercial products disrupts the microbial community, causes inflammation, and gives harmful bacteria an opening to colonize the area. The vagina is self-cleaning, and introducing fluids or solutions washes away the very lactobacilli that keep it healthy.
Some common products are directly harmful to protective bacteria. Lab studies have shown that certain vaginal moisturizers and spermicides can suppress lactobacillus growth within two hours and kill them entirely within 24 hours. Scented soaps, bubble baths, and intimate washes applied inside the vagina carry similar risks. Cleaning the external vulva with mild, unscented soap is fine, but nothing needs to go inside the vaginal canal.
Hormonal Changes
Estrogen plays a key role in maintaining the vaginal environment. It fuels the growth of lactobacilli and helps keep vaginal pH in the protective acidic range. When estrogen drops, pH rises, and the conditions become more favorable for BV-associated bacteria.
This is why BV risk can fluctuate with the menstrual cycle. Vaginal pH tends to be highest during and just after a period, when estrogen is at its lowest and menstrual blood (which is alkaline) further raises pH. Many women notice BV symptoms flare around their period for this reason. Menopause brings a more sustained estrogen decline. Postmenopausal women often have vaginal pH levels in the 6.0 to 7.5 range, well above the protective threshold of 4.5, which helps explain why BV and other vaginal infections become more common after menopause.
Other Risk Factors
- Antibiotics: Broad-spectrum antibiotics kill bacteria indiscriminately, including protective lactobacilli. This is why some women develop BV during or after a course of antibiotics for an unrelated infection.
- Smoking: Cigarette smoking is consistently linked to higher BV rates. Nicotine and its byproducts have been found in cervical mucus, and smoking appears to reduce lactobacillus populations.
- IUDs: Intrauterine devices, particularly copper IUDs, have been associated with a slightly higher BV risk in some studies, possibly because they can alter the vaginal environment.
- Stress and immune function: Anything that suppresses immune function can make it harder for the body to maintain its microbial balance.
Recognizing the Symptoms
About half of women with BV have no symptoms at all. When symptoms do appear, the most common are a thin, milky discharge that coats the vaginal walls and a noticeable fishy smell, often stronger after sex or during a period. Some women experience mild itching or burning, though intense itching is more typical of a yeast infection than BV.
Clinicians diagnose BV by checking for at least three of four signs: the characteristic thin discharge, vaginal pH above 4.5, the presence of “clue cells” (vaginal cells coated in bacteria) under a microscope, and a fishy odor. You won’t be running these tests at home, but knowing the typical symptoms helps you distinguish BV from other conditions. A grayish, fishy-smelling discharge is the clearest signal.
Why BV Keeps Coming Back
BV is notoriously recurrent. More than half of women treated for BV experience another episode within 12 months. The likely reason is that treatment kills the overgrown anaerobes but doesn’t fully restore the lactobacillus population. If the protective bacteria don’t recover quickly enough, the same imbalance can develop again. Ongoing exposure to the same risk factors, whether that’s a sexual partner who carries the bacteria, continued douching, or hormonal fluctuations, makes recurrence even more likely.
BV During Pregnancy
BV during pregnancy is associated with a higher chance of preterm delivery, meaning birth before 37 weeks. Preterm birth carries serious risks for the baby, including breathing problems and other complications. The relationship between BV and preterm delivery is well established statistically, though researchers are still working out whether BV directly causes early labor or is a marker for other underlying factors. If you’re pregnant and notice BV symptoms, getting it evaluated promptly is worthwhile.
Reducing Your Risk
Because BV stems from disruption of a natural ecosystem, prevention focuses on protecting that ecosystem. Avoiding douching is the single most impactful step. Using condoms with new or multiple partners reduces the bacterial disruption that comes with unprotected sex. Choosing unscented soaps for external washing and keeping products out of the vaginal canal helps preserve lactobacillus colonies. For women with frequent recurrences tied to their menstrual cycle, some clinicians recommend vaginal probiotics containing lactobacillus strains, though evidence on their effectiveness is still mixed.
Wearing breathable cotton underwear and avoiding prolonged time in wet swimwear are common recommendations, though their impact on BV specifically is less well studied than the factors above. The core principle is simple: the vagina maintains its own balance, and the less you interfere with that process, the lower your risk.