Can anyone tell me what causes bacterial vaginosis?

Vaginal flora: Bacterial vaginosis is a mild infection of the vagina caused by native bacteria. Normally, there are a lot of "good" bacteria and some "bad" bacteria in the vagina. The good types help control the growth of the bad types. In women with BV, the balance is upset. There are not enough good bacteria and too many bad bacteria.A sign of bacterial vaginosis can be a "fishy" smell, which may be worse after sex. About half of women who have bacterial vaginosis do not notice any symptoms.