The vaginal microbiome is the community of microorganisms that naturally live in the vagina, normally dominated by protective lactobacilli bacteria. These keep the environment slightly acidic, which helps hold harmful microbes in check.
When this balance is disrupted — by douching, antibiotics, hormonal changes, or sex — the protective bacteria can fall and others overgrow, making bacterial vaginosis and yeast infections more likely. Falling estrogen after menopause also shifts the microbiome (see vaginal health after menopause).