Bacterial vaginosis is not officially classified as a sexually transmitted infection, but the relationship between BV and sex is more complicated than a simple “no.” BV rarely occurs in people who have never had sex, and having new or multiple sexual partners is one of the strongest risk factors. Recent research has even shown that treating male partners can cut recurrence nearly in half. So while BV doesn’t fit neatly into the STI category, sexual activity clearly plays a central role.
Why BV Isn’t Classified as an STI
A classic STI is caused by a specific pathogen passed from one person to another during sex. Chlamydia, gonorrhea, and syphilis all follow this pattern: one organism, one clear transmission route. BV doesn’t work that way. Instead of a single infectious agent, BV involves a shift in the entire bacterial community of the vagina. Protective bacteria (primarily Lactobacillus species) decline, and a mix of other organisms fills the gap, most commonly Gardnerella, Prevotella, and Atopobium.
Because BV is a disruption in bacterial balance rather than an infection by one specific germ, it can develop without sexual contact. Anything that shifts vaginal pH or disturbs the microbial environment can contribute. Frequent douching, for example, has been linked to roughly double the risk of developing BV. This is part of why the CDC categorizes BV as a “vaginal condition” rather than an STI, even though it appears on the same pages as other sexually transmitted infections.
The Sexual Connection Is Strong
Despite the official classification, the evidence linking BV to sexual activity is hard to ignore. BV rarely affects people who haven’t had sex. New partners, multiple partners, and unprotected sex all increase the risk. The bacteria associated with BV have been found on penile skin and in the urethra of male partners, and studies in women who have sex with women show that partners frequently share the same BV-associated bacterial strains.
The strongest recent evidence comes from a landmark trial published in the New England Journal of Medicine. Researchers gave male partners a week-long course of oral and topical antibiotics alongside their female partner’s standard BV treatment. Within 12 weeks, BV recurred in 35% of women whose partners were treated, compared to 63% of women whose partners received no treatment. The trial’s safety board actually stopped the study early because the benefit of partner treatment was so clear. This finding strongly suggests that bacteria are being passed back and forth between partners during sex, even if BV itself isn’t a traditional “infection” one person gives another.
What Actually Happens in the Vagina
A healthy vaginal microbiome is dominated by Lactobacillus bacteria, which produce lactic acid and keep the environment acidic (below a pH of 4.5). This acidity acts as a natural defense, suppressing the growth of harmful organisms. In BV, Lactobacillus populations collapse. Without them, the pH rises and a diverse community of anaerobic bacteria takes over.
This shift is what produces the hallmark symptoms: a thin, white or grayish discharge and a noticeable fishy odor, which tends to be strongest after sex. Some people with BV have no symptoms at all and only learn about it during a routine exam. Clinicians diagnose BV by checking vaginal pH (above 4.5 is a flag), looking for characteristic “clue cells” under a microscope, and performing a whiff test where a chemical solution is added to a sample to detect the fishy smell.
Why It Raises Your STI Risk
Even though BV isn’t an STI itself, having it makes you significantly more vulnerable to actual STIs. The loss of protective Lactobacillus bacteria and the rise in vaginal pH weaken the body’s natural barrier against infection. A large meta-analysis of over 30,000 women found that those with BV had roughly 1.6 times the risk of acquiring HIV compared to women without it. Women with a highly diverse vaginal bacterial community (the kind seen in BV) had up to a 4.4-fold increased risk of HIV acquisition compared to women with a Lactobacillus-dominant microbiome.
BV also increases susceptibility to chlamydia, gonorrhea, and herpes. For pregnant women, untreated BV raises the risk of preterm birth and other complications. These downstream risks are one of the main reasons BV is worth treating even when symptoms are mild.
Treatment and the Recurrence Problem
BV is treated with antibiotics, typically a seven-day course of oral metronidazole or a vaginal cream (either metronidazole gel or clindamycin cream). Treatment usually clears symptoms within a week or two.
The real challenge is recurrence. Between 50% and 80% of women experience BV again within a year of completing antibiotic treatment. This extraordinarily high recurrence rate is one reason researchers have suspected a sexual transmission component for years. If the bacteria responsible for BV persist on a partner’s body, reinfection after treatment becomes almost inevitable.
Current CDC guidelines state that male partners do not need treatment. However, the 2024 partner-treatment trial may eventually change this recommendation. If you’re dealing with recurring BV in a relationship with a male partner, it’s worth discussing concurrent partner treatment with your healthcare provider, since the evidence now supports it even if guidelines haven’t yet caught up.
Reducing Your Risk
Because BV sits in a gray zone between an STI and a simple imbalance, prevention involves both sexual and non-sexual strategies. Condom use reduces BV risk by limiting the introduction of new bacteria. Avoiding douching is one of the most straightforward steps you can take, since douching disrupts the vaginal pH that keeps protective bacteria thriving. Scented soaps, sprays, and vaginal deodorants can cause similar disruption.
There’s no probiotic supplement proven to prevent BV, though research on Lactobacillus-based treatments is ongoing. For now, the most effective approach is minimizing the things known to throw off vaginal flora: douching, new barrier-free sexual exposures, and unnecessary vaginal products. If you’re experiencing recurrent episodes, tracking potential triggers alongside your provider can help identify patterns specific to your situation.