A prostitute goes to the hospital to visit a colleague who is about to 
have a heart transplant. She's worried about the friend so she asks the 
doctor: Girlfriend: I'm worried about my friend doc, what if her body 
rejects the organ?
Doctor: Well she's 36 years old and healthy. How long has she been in 
business?
Girlfriend: She's been working since she was 19 years old but what does 
that have to do with anything?
Doctor: Well she's been working 17 years and hasn't rejected an organ yet!