A joke is not a thing but a process, a trick you play on the listener's mind. You start him off toward a plausible goal, and then by a sudden twist you land him nowhere at all or just where he didn't expect to go.
“People are beginning to understand there is nothing in the world so remote that it can't impact you as a person. It's not just diseases. Economists are now beginning to say if we are going to have good markets in Africa, we're going to have to have healthy people in Africa.”