Old lady meets up with a Doctor at the local hospital for a tour.  The Doctor opens the first door and the guy on the bed is jacking off.  The Lady says "Oh my! What is going on in here?"  The Doctor says "He has a disease in his nuts and must masterbate all the time or his nuts will explode".  They walk on to the next room and open the door.  There is a Beautiful, Blonde, Busty nurse with a very short skirt on giving the guy a blowjob.  The old lady says "My oh my what is going on in here?"  The Doctor looks at his chart and says "Same disease, better insurance".

Ashley beat you to the punch; she told that joke here a couple of months ago; but it is a really good one, so it deserves repeating!

dang, didn't know it was posted before.  Of course Ashley would have.  Now have to think of another.