@sozobe,
Normally, the vagina is sufficiently acidic that the "yeast" (i forget the latin name) cannot culture there. The microrganism is always present, however, and usually is present in the vulva. If anything occurs which alters the pH balance of the vagina, an infection can arise, in that the mircorganism is able to "culture" itself--it can start a colony. Although it may be possible that the type of underwear could affect a change in the pH of the vagina, it beats me how that would happen. One of the most common causes of an altered pH balance in the vagina is a particularly heavy menstrual flow.
EDIT: This is what i picked up working in the Army Medical Corps. I would advise Gracie to read up on the subject.
