A man was very worried that his wife was lately not quite the same as she used to be. So, he takes her to the doctor who runs a battery of tests on her and then calls the man into his office.
He says, "Well, I have run all the tests on your wife and have come to the conclusion that she has either Aids or Alzheimers."
The man is now shocked and upset and asks the doctor, "But doc, how will we know which one of the diseases she is suffering from?"
The doctor replies, "On your way back home, drop her off in a place about two blocks from home. If she makes it home, don't have sex with her!!"