AIDS or Alzheimers?

An elderly couple goes to the doctor's wondering what is wrong with the
husband.  After an intense and exhausting examination, the doctor
decides to tell the wife in private.

"Well", he says.  "We've narrowed it down to two things.  Unfortunately,
your husband has either AIDS or Alzheimer's."

"Ohmigod," gasp the elderly woman.  "Those are very serious indeed.  Is
there anyway you can find out for sure?"

"Actually yes there is," replies the doctor.  "On your way home tonight,
drop your husband off 10 miles from home.  If he comes home, don't fuck
him."