Q: What did one elevator say to the other elevator?
A: I think I’m coming down with something!

Explanation:  There is a double meaning in this joke: “To come down with something” is to get sick; elevators come down with something- people.  When the first elevator says it is coming down with something the listener hears the double meaning of getting sick and going to a lower floor — and that’s funny because elevators don’t get sick.