On this day, February 16th 600 A.D., a papal decree was issued by none other than Pope Gregory I the Great.
It stated that all good Christians would give a blessing when they heard another person sneeze, by saying: "God bless you"
It was the running idea at the time that sneezes would spread the plague (This was the Justinian Plague), which to be fair to them, was a pretty solid theory. Sneezes are a classic vector for bacteria to spread from person to person. The remedy of just bombarding the sneezer with blessings, however, may leave much to be desired from the perspective of modern medical science.
Funny to think though, that the tradition of saying "Bless you" to someone who sneezes is 1,400 years old, and will probably continue on until we all stop speaking English.