Doctor : How did you get that bump on your head?

Patient : A book fell on it

Doctor : Oh, don't blame your shelf.

or

Doctor : You've got chicken pox

Patient : Chicken pox! I want a second opinion.

Doctor : Oh alright, it's measles.

(courtesy T.Cooper)

or

Did you hear about the girl who went into a bar and asked for a double entendre, so the barman gave her one?

Boom boom.