The original term, "May God bless you" was contracted into "God bless you," or sometimes just "God bless." The verb "bless" is used in this phrase instead of "blesses" because it is in the...
In short, no-one knows. A common story is that during the time of the bubonic plague (of which sneezing was an early symptom) people would say "bless you" in case the sneezer had the plague - the...