My understanding is that the human digestive system is too harsh (acids and enzymes and the like) for the virus to survive long enough to infect you. On the other hand, we're talking about an entirely fictitious realm here and we can make up what ever details we like. So let's assume that being dead (albeit "living" dead) for a couple hundred years and having a strictly hemotalogical diet has an effect on one's digestion. It would stand to reason that an unsuspecting vampire could fall victim to AIDS in this way. How sad.