also bo21 there is a thing called a dental dam. it's like condom material that you can put between your mouth and her vagina to protect you and her as well.
if you use the dental dam, it keeps your saliva from coming in contact with her vagina and it keeps her vaginal secretions from coming in contact with your mouth.
you still get to turn her on and stimulate her with your tongue through the dental dam, so maybe this is a good option for you if you are concerned.
also, if you are doing this regularly with more than one partner, I would highly suggest a dental dam or some form of barrier to protect yourself, and PARTICULARLY the women you are having oral sex with. if you are doing this regularly, with several partners, you are more likely to spread disease than anyone.
jasmine