Small cuts or scrapes in the vagina or vulva can allow yeast to invade.
Tampons, especially the type with extremely high absorbency, can sometimes cause microscopic cuts. Women who get frequent yeast infections may want to use unscented pads for a while to see if it helps.
Sex and the use of sex toys without enough lubrication can also cause tiny abrasions of the vaginal wall. These small injuries can lead to yeast infections.
Some people recommend that you always use unscented sanitary pads instead of tampons to avoid damaging the membranes of the vaginal canal. Others have pointed out that the pads will keep your crotch from breathing, and this can create a good environment for yeast. Your own experience should tell you which advice is more helpful for you. If you do use pads, you’ll want to change them frequently (so the yeast doesn’t take up housekeeping in the pads), and give your crotch some fresh air whenever possible.