The human body is host to many different microbial micro-environments, and the vagina is no exception. The healthy human vagina is quite acidic, with a pH less than 4.5, similar to that of grapes or orange juice. This pH is maintained by a healthy population of Lactobacilli which produce lactic acid and hydrogen peroxide. These products of Lactobacillus metabolism appear to keep in check populations of other vaginal bacteria, many of which are intolerant of hydrogen peroxide and of acidic environments.From time to time, this normal balance of bacteria is disrupted. When this happens, the normally dominant Lactobacilli are outgrown by various anaerobic bacteria. These bacteria break down proteins in the vagina and create various malodorous compounds that create a thin, grey discharge. It is this symptom that normally drives a woman to the doctor where she is diagnosed with bacterial vaginosis (BV).
As the vaginal environment changes and the pH rises, Garnerella bacteria begin to stick to vaginal epithelial cells and are visible under the microscope as "clue cells" (the big blobs are squamous cells, the little dots are bacteria stuck to the surface).
BV is not a sexually transmitted disease, but it is strongly associated with sexual activity. Sexual activity with men or women increases the risk of BV, as does douching. Both sexual activity and douching can change the normal vaginal environment, and one of these activities can be safely and comfortably done away with.
Normally, BV is a benign condition, but in pregnant women it can increase the risk of premature delivery. It is also a risk factor for acquiring STIs such as HIV, chlamydia, and herpes, perhaps because the normal acidic and oxidizing environment is protective. Because of these risks, BV should be treated, and treatment is relatively easy, although relapse is common.
Because strains of Lactobacilli are present in many yogurts, yogurt has been touted as a possible prevention and treatment for BV, taken either orally or intravaginally, but there are many different species in this genus, and only some of them produce hydrogen peroxide, a trait thought to protect the vagina. One review found some evidence that yogurt can be helpful, but most of the studies out there don't compare it to the gold standard antibiotic.
The vagina and its microflora do a pretty good job staying healthy, but certain human activities can create significant problems. Douching is never necessary and can lead to BV. It's a bad idea. While giving up douching should be pretty easy, I'm fairly certain people are still going to have sex. When symptoms of BV develop, a simple gynecologic exam, including checking vaginal pH and looking at secretions under the microscope, leads to rapid diagnosis and treatment of an annoying and potentially harmful disorder.