PalMD has a great post about vaginas. More specifically, about the wonderful commensal bacteria that help keep a vagina at the proper pH, and what happens when they get booted out:
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 are discharged in a thin, grey discharge. It is this symptom that normally drives a woman to the doctor where she is diagnosed with bacterial vaginosis (BV).
If that doesn't convince you to go read the whole thing, I don't know what will.