People get infected with STDs, (this includes HIV
the virus that can lead to AIDS
) by having unprotected insertive vaginal/anal sex.
Weather her vagina is dry or not, still puts one at risk for STDs.
Finally, to answer your last question, ADIS comes from a patient being untreated for HIV infection. Who of course either had unprotected vagainal/anal sex with an infected partner. Shared needles with an infected person, or it was passed on from mother to child.