What is the basis for saying "God bless you" when a person sneezes?

It is common among many Christians to say "God bless you" (or simply "bless you," which implies the former) to a person when they sneeze. I have heard that this may have originated with an old belief that evil spirits were coming out of a person when they sneezed. Is there any basis (scriptural or otherwise) within Christianity for saying "God bless you" when a person sneezes?