God bless you: Interjection used especially when someone has sneezed. In this situation, "God bless you" is more or less equivalent to saying "Gesundheit."
In German "Gesundheit" means "health" (and "sanity"). Wishing "good health" was once thought to hold off the illness that might follow a sneeze.
"God bless you" came into usage to prevent the escape of the soul from the body, which was thought to occur with a sneeze. (Soullessness was believed to cause ill health.)