BV is caused by a shift in the vagina's bacterial balance: the protective Lactobacillus that normally dominate decline, and other bacteria — chiefly Gardnerella and other anaerobes — overgrow. The environment becomes less acidic, which produces the thin discharge and fishy odor. It is not caused by poor hygiene and isn't something you catch from a toilet seat.
BV doesn't arrive from the outside. It's the result of a balance shifting inside an ecosystem you already carry — a community of bacteria that lives in the vagina and usually keeps itself in check. When that balance tips, the symptoms follow. Understanding why it tips is the most useful thing you can know about BV, because it explains both the odd timing of an episode and what actually raises or lowers your odds of another.
Here is what's really happening at the level of the bacteria, the everyday things that nudge the balance, and the two stubborn myths worth letting go of.
The balance that tips
The healthy vagina is not sterile, and it isn't meant to be. It hosts a stable community of bacteria dominated by Lactobacillus. These bacteria do something quietly protective: they produce lactic acid, which keeps the vaginal environment acidic — typically a pH at or below 4.5. That acidity is hostile to most other organisms, so the Lactobacillus effectively hold the territory and crowd everyone else out.
BV is that arrangement breaking down. For reasons that aren't always obvious, the Lactobacillus populations decline. As they thin out, the acidity they maintained fades and the pH drifts upward, past 4.5. That less-acidic environment is suddenly hospitable to a different set of bacteria — anaerobes that prefer low-oxygen conditions, chiefly Gardnerella vaginalis along with several others. Freed from the competition, they multiply into a dense mixed population, often organized into a biofilm that clings to the vaginal wall.
That overgrowth is what you notice. The anaerobes produce volatile compounds called amines, and amines are what create the characteristic fishy odor — which is why the smell sharpens after sex or during your period, when semen or menstrual blood (both less acidic) push the pH up further still. The thin, grayish discharge comes from the same shift. If you want to see how those signs present, our guide to BV symptoms walks through each one.
The single idea worth holding onto: BV is not an invasion. The bacteria involved were already present, or arrived without fanfare. What changed is the balance that was keeping them quiet.
Common triggers
If BV is a balance tipping, the practical question is what tips it. Researchers don't have a single cause, but a consistent set of factors disturb the protective Lactobacillus or otherwise shift the environment. The recognized ones:
- Douching. The most clearly avoidable trigger. Rinsing the vagina with water or commercial products washes out protective bacteria and disrupts the acidic balance — doing exactly the opposite of what people intend.
- A new or multiple sex partners. A recent change in partner, or more than one partner, is strongly associated with BV. Sexual activity appears to disturb the vaginal balance, even though BV itself isn't classed as a sexually transmitted infection.
- Sex without condoms. Semen is alkaline, and unprotected sex is linked to higher BV rates. Condom use is associated with a lower risk.
- Menstruation and hormonal shifts. Blood raises vaginal pH, which is why symptoms often flare around a period; broader hormonal changes may play a part as well.
- Smoking. Smoking is consistently associated with BV, likely through effects on the vaginal bacterial community.
- An intrauterine device (IUD). Some studies link IUD use to a higher rate of BV, though the relationship isn't fully settled.
You won't always be able to point to a trigger, and that's normal — sometimes the balance simply shifts on its own. But when BV keeps returning, this list is where a clinician starts looking. Our guide to recurrent BV takes up that pattern, and the prevention guide covers which of these factors you can actually act on.
Not about hygiene
Two beliefs about BV are common enough — and wrong enough — to address head-on, because both tend to produce needless shame.
BV is not caused by poor hygiene. If anything, over-cleaning makes it more likely. Douching, scented washes, and harsh soaps strip away the protective Lactobacillus and disturb the acidity that keeps anaerobes in check. The vagina is self-cleaning; aggressive washing works against the very balance you're trying to protect. Plain water on the external skin is all that's needed.
It also isn't something you pick up from the environment. You can't catch BV from a toilet seat, a swimming pool, a hot tub, or shared bedding. The bacteria involved aren't lurking on surfaces waiting to colonize you — they're a shift within your own resident community.
This is also a useful place to mark the line between BV and a yeast infection, which people routinely confuse. A yeast infection is an overgrowth of a fungus rather than a shift in bacteria, and its causes run differently — antibiotics, high-estrogen states, and uncontrolled blood sugar are the usual drivers there, not the bacterial imbalance behind BV. The two even move pH in opposite directions, which is part of why telling them apart matters before reaching for a treatment.
An occasional bout of BV is routine and not a worry on its own. But see a clinician in person if you have fever or pelvic pain, think you might be pregnant, have had a possible STI exposure, or are getting repeated episodes — these point to causes worth evaluating directly rather than treating blind.
None of this is cause for alarm. For most women BV is a treatable, often one-off event tied to a passing disturbance in a normally stable system. Knowing the real cause means you can stop blaming the wrong things — and focus on the few factors that genuinely move the odds.