With the caveat that typing a joke robs it of some of the humour...
Old Schlomo Berkowitz is an old Jewish businessman and he is very ill. He goes to a number of doctors, and yet none of them can figure out whats wrong with him.
Finally, he goes to a specialist and the specialist assures Mr. Berkowitz that he will figure out whats wrong with him. He subjects him to an exhaustive series of tests, they take weeks, and at the end of it Schlomo feels even more drained than before. He says to the doctor, "Doc, can you tell me what's wrong with me?"
The doctor says, "Let me ask you: Is Mrs. Berkowitz here?"
"Of course, she's right out in the waiting room."
"Ok - I want you to go out and send your wife in. You wait out in the waiting room."
So Mr. Berkowitz goes out and sends his wife in. Mrs. Berkowitz is very upset when she comes in to see the doctor.
"Doc, do you know what's wrong with my husband?"
"Yes, I think I've figured it out. Your husband has a very, very rare genetic disorder, and without the cure he will surely die."
"Doctor," she says, "What's the cure? We're very wealthy, we'll pay anything, how can we save my husband?"
"The cure," says the doctor, "is oral sex."
"What?"
"Oral sex. You have to give your husband oral sex or he's going to die."
"What?!"
"Do you know what oral sex is?"
"Yeah, yeah, I know" says Mrs. Berkowitz and she thanks the doctor and goes back out into the waiting room.
"Honey, honey!" says Mr. Berkowitz, "What did the doctor say? What's wrong with me?"
"I don't know what to tell you," says Mrs. Berkowitz, "You're gonna die."