Causes and Risk Factors of Bacterial Vaginosis

How Sexual and Health Practices Contribute to Risk

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Bacterial vaginosis (BV) is the most common vaginal infection in women of reproductive age and one of the most misunderstood.

BV is associated with the same risk factors as the sexually transmitted infections (STIs) chlamydia, gonorrhea, and trichomoniasis. In fact, scientists aren't even entirely sure which mechanisms give rise to BV or why some women are prone to the infection and others aren't.

What is known is that, whatever the underlying cause, BV is the result of an imbalance in the vaginal flora, the bacteria that live inside the vagina. When an imbalance occurs, healthy bacteria are depleted, allowing unhealthy ones to overgrow. Some of the possible triggers are sexual practices, genetics, and general/vaginal health.

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Common Causes

BV infection occurs when certain "bad" bacteria commonly found in the vagina are given the opportunity to thrive.

The culprits include Gardnerella vaginalis, Atopobium vaginae, and strains of the Prevotella and Morbiluncus bacteria. These bacteria are usually kept in check by the immune system and, more importantly perhaps, the acidity of the vagina (as measured by the vaginal pH).

It is not clear whether BV should be considered an STI, but some believe that Gardnerella can be transmitted from one partner to another.

Sexual intercourse can disrupt the vaginal flora by introducing new microbes into the vagina. This may not only alter the vaginal pH, it can strip away many of the healthy bacteria that support and "clean" the vagina.  Microbes can be introduced each time you have intercourse.

The risk of BV is highest among people with a vagina between the ages of 15 and 44. This age group tends to be more sexually active than younger or older people. Bacterial vaginosis is rare in people who have never been sexually active.

In addition to BV, women can develop what is called a mixed infection, the presence of two different enzymes. Mixed infection occurs when the vagina is exposed to anaerobic bacteria commonly found in the vagina as well as aerobic bacteria that are foreign to the vagina. Examples of aerobic bacteria include Staphylococcus aureus and Escherichia coli (E. coli).


In some cases, a person's genetics may contribute to their BV risk, usually by causing lower-than-expected levels of protective lactobacilli in the vagina.

While the research is nowhere near conclusive, there is evidence that certain genetic mutations may affect the production of corticotropin-releasing hormone (CRH), which plays an important role in regulating immunity and inflammation. Scientists believe that abnormalities in CRH production may affect vaginal tissues and trigger an imbalance in bacteria populations, especially during pregnancy.

A number of CRH-related genetic mutations have been identified in black women that are less common in white women. This may help explain, in part, why black women are twice as likely to be diagnosed with BV than their white counterparts.

Lifestyle Risk Factors

Sexual practices, vaginal health, and general health all play a part in establishing your personal risk of bacterial vaginosis. All of the following factors are modifiable, meaning that you can change them and reduce your risk of infection:

Sexual Risk Factors

While bacterial vaginosis is not an STI, it shares many of the same characteristics of infections that result from sexual activity. Key among these are:

  • Having multiple sex partners is one of the major risk factors of BV. This includes both male and female partners. In fact, a 2010 study concluded that having sex with another woman increases your risk of BV by as much as 52%.
  • New sex partners pose a risk simply by introducing you to bacteria and other microorganisms your body may not be unaccustomed to.
  • Unprotected oral, vaginal, and anal sex contribute by removing the barrier of protection that condoms and dental dams provide. BV can also be caused by manual sex (masturbation, "fingering") and frottage ("dry humping").
  • Shared sex toys also pose a potential risk.

In terms of vaginal health, maintaining the optimal pH and flora is not always easy. Many hygiene practices can undermine this delicate balance, either by promoting the overgrowth of "bad" bacteria or impairing our ability to fight infection.

Other Risk Factors

Among the practices or conditions most associated with a BV infection:

  • Douching places you at risk by stripping the vagina of its protective flora. According to a report from the Department of Health and Human Services, 1 in 5 American women ages 15–44 douche.
  • Smoking is known to deplete two bacteria vital to your vaginal health: Lactobacillus iners and Lactobacillus crispatus. Smoking also causes the constriction of blood vessels, making it harder to fight infection when increased circulation is needed.
  • Copper-containing intrauterine devices (IUDs), while effective in preventing pregnancy, can double the risk of BV in certain women. According to a study from the St. Louis School of Medicine, the risk appears greatest in people who have an underlying imbalance in their vaginal flora (often undiagnosed) and experience irregular bleeding while using an IUD.

By better understanding the risks of bacterial vaginosis, you can find the means to prevent it and avoid other, more serious sexually transmitted infections.

Frequently Asked Questions

  • What causes bacterial vaginosis?

    Bacterial vaginosis (BV) is caused by an imbalance in the vaginal flora that allows unhealthy bacteria to proliferate. While scientists haven't pinpointed an exact cause, BV can be the result of several different potential risk factors, such as douching, smoking, using certain IUDs, and having multiple sex partners. Genetics also plays a big role.

  • Is BV a sexually transmitted infection?

    No, BV is not considered an STI because it is not technically caused by a pathogen, such as a virus or bacterium. BV simply develops when "bad" bacteria that are often found in the vagina overgrow and throw off the delicate flora balance.

  • Does BV only affect women?

    BV affects all people with vaginas. However, people with penises can cause the bacterial imbalance, without becoming infected.

  • Why might bacterial vaginosis keep coming back?

    Unlike some infections, BV can recur. In some cases, it can be a chronic condition. If you have symptoms, it's a good idea to seek treatment and take the full course of antibiotics to clear the infection.

16 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  14. Office On Women’s Health. Douching.

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Additional Reading
  • Brotman, R.; He, X.; Gajer, P. et al. Association between cigarette smoking and the vaginal microbiota: a pilot study.BMC Infect Dis. 2014; 14:471. DOI: 10.1186/1471-2334-14-471.

  • Madden, T.; Grentzer, J.; Secura, G. et al. Risk of Bacterial Vaginosis in Users of the Intrauterine Device: A Longitudinal Study.Sex Trans Dis. 2012; 39(3):217-22. DOI: 10.1097/OLQ.0b013e31823e68fe.

  • Ryckman, K.; Simhan, H.; Krohn, A. et al. Predicting risk of bacterial vaginosis: the role of race, smoking and corticotropin-releasing hormone-related genes.Mol Hum Reproduction. 2009; 15(2):131-137. DOI: 10.1093/molehr/gan081.

  • Taheri, M.; Baheiraei, A.; Foroushani, A. et al. Treatment of vitamin D deficiency is an effective method in the elimination of asymptomatic bacterial vaginosis: A placebo-controlled randomized clinical trial.Indian J Med Res. 2015; 141(6):799-806. DOI: 10.4103/0971-5916.160707.

  • Turner, A.; Carr Reese, P.; and Fields, K. A blinded, randomized controlled trial of high-dose vitamin D supplementation to reduce recurrence of bacterial vaginosis.Am J Obstet Gynecol. 2014;211(5):479.e1-479.e13. DOI: 10.1016/j.ajog.2014.06.023.

By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.