Vaginosis is a general medical term for non-sexually-transmitted bacterial infections of the vagina; they are the most common vaginal infection (more common, apparently, than even yeast infections).
The condition typically occurs when there's an overgrowth of bacteria normally found in the vagina due to a decrease of the pH of the vagina or because a woman's immune system is compromised. It can also occur when more pernicious bacteria from the intestinal tract are introduced into the vagina. Gardnerella is the most common species to cause vaginosis, but other anaerobic bacteria and spirochetes may also cause it.
Symptoms include bad odor (often "fishy") and icky gray or white discharge; there may also be itching, sometimes severe. However, up to 40% of women with vaginosis may have no noticeable symptoms. If left untreated, vaginosis can lead to pelvic inflammatory disease; it has also been implicated in the development of cervical cancer.
Treatment typically involves giving an afflicted woman antibiotics, usually metronidazole (Flagyl), which may be given as a pill or topical cream. Women who are prone to vaginosis may be able to keep themselves healthy by avoiding perfumed feminine hygiene sprays and douches and by wearing breathable cotton underwear. Women may also gain good results by taking lactobacillus supplements or eating yogurt; lactobacilli are important in maintaining the pH of the vagina, and their presence discourages the growth of other bacteria.
The National Vaginitis Association says that 12%-25% of women undergoing routine gynecological exams have been found to be suffering from bacterial vaginosis; the rate for women seeking treatment for sexually transmitted diseases is 32%-64%.
References: http://familydoctor.org/handouts/234.html and http://www.vaginalinfections.com/