Verbally responding to a person's sneeze with phrases like "Bless you," "God bless you," and "God bless" is as expected as saying nothing when they cough. And it's just as expected to say "thank you" to the person who is blessing you. But when did our culture began doing that? And what does it really mean?
According to HowStuffWorks.com, it may have started during the Roman Empire with people saying "Jupiter preserve you" as a way to wish someone good health despite whatever was causing them to sneeze. During the bubonic plague of the 6th century, Pope Gregory the Great is said to have altered it to "God bless you" as a way to help bestow health on the sneezer in the hopes they wouldn't soon die from the plague.
Snopes.com presents several other theories, from protecting someone's soul after their sneeze jarred it loose from their body, to protecting the sneezer from being possessed again by a demon that was ejected from their soul by a sneeze, to encouraging the sneezer's heart to start beating again after the sneeze, or as way of acknowledging the good luck that the sneeze was surely to impart upon the sneezer.
Of all of these origin theories, though, Snopes has determined the correct answer to be: undetermined.
Do you have other explanations?