As with many common phrases that have been around for seemingly forever, there are a lot of possible stories about its origin.
The Plague of Justinian
The Plague of Justinian was a particularly nasty plague that hit Europe hard during the late 6th century. Europe was dominated by the Roman Catholic Church at the time, and its leader, Pope Gregory I directed adherents to fight the Plague spiritually, ordering consistent prayer to ask for divine help in stopping the plague. The first defense in Pope Gregory’s plan was to invoke the assistance of God when a person showed an early symptom of the plague. That symptom was sneezing, and enlisting divine intervention meant saying “God bless you.” Within 150 years, long after the Plague of Justinian was over, it was considered polite in Europe to say “God bless you” upon any sneeze.
Ancient European Folk Legends
But Pope Gregory’s order may be rooted in ancient European folk legends. Historians cite several cultures in Europe that held the belief that sneezing opened up the body to allow for demons or evil spirits to invade or take over, and that invoking a god acted as an immediate deterrent to those nasty forces.
Another theory says that there’s a medical reason for saying “God bless you.” Before the advent of modern medicine could prove otherwise, some cultures in medieval Europe believed that sneezing was such a strain, if a momentary one, that the heart stopped beating for a second. It would continue beating again, the theory goes only if someone said “God bless you.”
But not everybody says “bless you” in response to a sneeze. Sure, some people say nothing at all, while others utter the German word “gesundheit.” It’s a popular misconception that it’s the German word that means “God bless you.” It more accurately translates to “health.” Centuries ago in Germanic cultures, sneezing was considered a sign of divine blessing. Saying “gesundheit” is an acknowledgment of that good fortune.”