Image via WikipediaThe social habit of blessing someone after a sneeze came about during the reign of Pope Gregory the Great (approx. 550 AD), whose predecessor died at the start of the plague. In hopes of squelching its spread, Pope Gregory began praying in earnest.
He also sent people out to pray street-by-street. Each time a sneeze was heard, they called out, "God bless you," hoping to replace the plague with the blessing of health in the person. Did it work? It was one of the briefest plagues in history.
I found this at Take Root and Write a Christian Womans Blog