A woman goes to the doctor's office for a physical. The doctor takes a blood sample and tells her to return in one week for the results. One week later, her and her husband return to the doctor's office. The doctor takes the husband aside and tells him, "Sir, I'm afraid I have some bad news, we had mixed your wife's blood sample with another patient's and we have no idea whos is whos. The bad news is one has Alzheimer's disease and the other has AIDS. I want you to come back in another week and by then I should have it all sorted out." The man lookes scared and says, "That's terrible doc, what should I do until then?" "Well, when you're driving home today, drop her off 2 blocks away from your house, if she makes it home, DON'T HAVE SEX WITH HER!!"