According to an article in Smithsonian magazine a few years back, the phrase originated when it was decreed as mandatory by the Roman Catholic Church during the Plague Epidemic in Europe. The church, it is said, mandated that "God bless you" (an "invocational blessing" in ecclesiastical terminology) be said over anyone who sneezed, in the belief that an evil spirit might be communicated between the plague victim and anyone standing nearby. As was noted earlier, the phrase in German is "Gott segne dich", which sounds more like a command than a prayer, but is the standard form in German.
"God bless you"
To prevent "rigor mortis," I think I'll settle for "Gesundheit." (Which also has its roots in the plague times.) :D