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Usually, there are a lot of "friendly" bacteria called lactobacillus in your vagina. They help keep your vagina healthy. If you have bacterial vaginosis, these "friendly" bacteria have been replaced with other bacteria called anaerobes.

Doctors aren't sure what causes bacterial vaginosis. But some things increase your chances of getting it.

You might be more likely to get bacterial vaginosis if you:^{1 2 3}

• Have a new male sexual partner
• Have sex with other women
• Have sex at an early age
• Have a lot of sexual partners
• Have, or once had, a sexually transmitted infection
• Use douches
• Smoke
• Have an IUD (an intrauterine device or contraceptive coil).

Women with bacterial vaginosis have many more types of bacteria in their vaginas than women without bacterial vaginosis.⁴ And these bacteria are different from the usual types. The vaginas of women with bacterial vaginosis are also less acidic than normal.¹ But we don't know why.

A lot of women seem to get bacterial vaginosis when they are having their period.⁵

Bacterial vaginosis seems to be linked in some way to having sex. But researchers don't think you can "catch" it from a male sex partner. The infection does not seem to pass from a man to a woman during sex.⁶

If you are a female sex partner of a woman who has bacterial vaginosis, some research shows you have a higher chance of being infected, too. But researchers aren't sure why.³

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.