I heard this one yesterday:

A man is expecting some test results for his wife when he gets a call from her doctor. The doctor tells him, "Well, Mr. Smith, I don't know how to tell you this, but it seems that your wife's test results got mixed up with another Mrs. Smith. I managed to track down the other results, but we don't know which ones belong to your wife and which belong to the other woman. So your wife either has Alzheimer's or AIDS."

Mr. Smith asked the doctor, "So do we have to re-do the tests then?"

To which the doctor replied, "Unfortunately, your insurance won't cover another set of tests. They did suggest a solution for the problem though."

"Well, what is it?"

"Drive your wife across town and drop her off on a street corner. If she makes it home, don't sleep with her!"

...

...That's the best I can do for now. xP