A guy goes to the doctor and tells the doctor he hasn't been feeling well - tired, losing weight, etc. The doctor runs a few tests, then tells the guy to come back the next day with a banana and two cookies. The guy is confused, but tells the doctor he'll do so.
The next day he arrives at the doctor's office with a banana and two cookies. The doctor has him take off his pants and bend over, and then he inserts the banana and two cookies up the guy's ass. He asks the guy to come back the next day with a banana and two cookies.
This repeats every day for almost a week. The guy arrives with a banana and two cookies, and the doctor inserts them up the guy's ass. Just as the guy is about to lose it, the doctor asks him to come back the next day with a banana and a hammer. The guy is obviously concerned, but the doctor asks him to trust him.
The guy comes back to the doctor's office the next day with the banana and a hammer. He pulls down his pants, bends over, and the doctor inserts the banana into his ass and stands there with the hammer. They wait for some time and the guy asks the doctor what's going on, but the doctor just tells him to wait.
Suddenly, a tape worm pops his head out of the guy's ass and says, "HEY! Where's my two cookies??"