Usually, "good" bacteria (lactobacilli) outnumber "bad" bacteria (anaerobes) in your vagina. But if anaerobic bacteria become too numerous, they upset the natural balance of microorganisms in the vagina and lactobacilli decrease.
Factors that can disrupt the balance of bacteria in the vagina include:
- Reacting to antibiotics
- Using an intrauterine device (IUD)
- Having unprotected sex with one or more partners