Bacterial vaginosis (BV) is not technically a sexually transmitted disease, but its relationship to sex is close enough to cause real confusion. BV is a vaginal imbalance where the bacteria that normally keep things healthy get crowded out by other types of bacteria. It’s the most common vaginal condition in women of reproductive age, and while sexual activity is strongly linked to it, scientists still haven’t confirmed that a single sexually transmitted pathogen causes it. Women who have never been sexually active rarely develop BV, which is part of why the line between “STD” and “not an STD” feels blurry.
Why BV Isn’t Classified as an STD
A true STD is caused by a specific pathogen passed from one person to another during sex. BV doesn’t fit that model cleanly. Instead of a single infectious agent, BV involves a shift in the entire bacterial ecosystem of the vagina. Protective bacteria (mainly Lactobacillus species) that produce lactic acid and hydrogen peroxide decline sharply, and a mix of anaerobic bacteria that are normally present in small numbers take over. The CDC describes BV as a “vaginal dysbiosis,” essentially a disruption of the microbial community rather than an infection caused by one invading organism.
That said, sexual activity is one of the strongest risk factors. Having new or multiple sexual partners increases your chances significantly, and not using condoms raises risk as well. The pattern looks a lot like transmission, which is why researchers continue to study whether BV-associated bacteria can be passed between partners during sex. The honest answer is that science hasn’t fully resolved the question yet.
What Causes the Bacterial Shift
A healthy vagina is dominated by Lactobacillus bacteria, which keep the environment slightly acidic (below a pH of 4.5). That acidity acts as a natural defense against harmful organisms. When Lactobacillus populations drop, the vaginal pH rises and becomes more alkaline, creating conditions where anaerobic bacteria can multiply rapidly. The result is a dramatic change in the vaginal flora, with dozens of bacterial species that were previously a tiny minority now making up most of the community.
Several things can trigger this shift:
- New or multiple sexual partners: Exposure to different bacteria can destabilize the existing microbial balance.
- Douching: Washing inside the vagina disrupts the protective bacterial layer and alters pH.
- Not using condoms: Semen is alkaline and can temporarily raise vaginal pH, and direct bacterial exchange may play a role.
Some people develop BV repeatedly despite avoiding all known risk factors, which underscores that the condition isn’t fully understood. Genetics, immune response, and the specific composition of someone’s baseline microbiome all likely contribute.
Recognizing the Symptoms
Many people with BV have no symptoms at all. When symptoms do appear, the most common sign is a thin, grayish-white to yellowish vaginal discharge with a noticeable fishy odor. The smell is often stronger after sex or during a period. The discharge tends to be uniform and watery rather than thick or clumpy, which helps distinguish it from a yeast infection.
Clinicians diagnose BV by checking for a few specific markers: vaginal pH above 4.5, the characteristic discharge, and a fishy smell that intensifies when the sample is exposed to a chemical solution. Some also examine a sample under a microscope to look for “clue cells,” which are vaginal cells coated with bacteria. If you notice an unusual smell or discharge, testing is straightforward and can be done during a routine visit.
Health Risks of Untreated BV
BV isn’t just a nuisance. The shift in vaginal bacteria weakens the body’s local defenses, making you more vulnerable to acquiring sexually transmitted infections, including HIV, chlamydia, and gonorrhea. BV can also increase the risk of pelvic inflammatory disease, a condition that can damage the reproductive organs and affect fertility.
During pregnancy, BV carries additional concerns. People with BV may be more likely to deliver before 37 weeks, which is linked to serious health problems for babies, including breathing difficulties, brain bleeding, and in severe cases, death. However, research hasn’t definitively established that BV directly causes preterm delivery, and routine screening for BV in pregnant people without symptoms remains a matter of debate among medical organizations.
How BV Is Treated
BV is treated with prescription antibiotics, typically taken orally or applied as a vaginal gel or cream. Treatment courses usually last about a week. Most people see symptoms resolve within a few days of starting treatment, though it’s important to finish the full course.
The real challenge with BV is recurrence. Roughly 58% of women experience BV again within 12 months of successful treatment, and up to 66% have a recurrence within a year by some estimates. That high recurrence rate has frustrated both patients and clinicians for years, because it means many people cycle through repeated rounds of antibiotics without a lasting solution.
The Evolving Role of Partner Treatment
For decades, treating male sexual partners wasn’t recommended because studies hadn’t shown a clear benefit. That changed in 2025, when the American College of Obstetricians and Gynecologists (ACOG) recommended concurrent partner treatment for recurrent BV for the first time. New research showed that sexual activity plays a significant role in both initial BV and its return, and that BV-associated bacteria can be harbored by male partners and reintroduced during sex.
This doesn’t mean every case of BV requires partner treatment. The recommendation applies specifically to people dealing with recurrent episodes, where the cycle of treatment and relapse suggests reinfection may be a factor. If you’ve been treated for BV multiple times and it keeps coming back, asking your provider about partner treatment is a reasonable conversation to have.
Reducing Your Risk
Because the exact cause of BV remains unclear, prevention isn’t guaranteed. But a few evidence-based steps can lower your chances. Avoiding douching is one of the most consistent recommendations, since it directly disrupts the vaginal microbiome. Using condoms reduces exposure to bacteria and buffers against the pH changes that semen causes. Limiting the number of new sexual partners also appears to reduce risk, though BV can occur in long-term monogamous relationships as well.
There’s no proven benefit from probiotics, special soaps, or vaginal supplements for preventing BV, despite widespread marketing claims. The vagina is largely self-cleaning, and the best approach is to avoid products and practices that interfere with its natural bacterial balance.