Gesundheit: Interjection used to wish "good health," especially to someone who has sneezed. In this situation, Gesundheit is roughly equivalent to "God bless you."
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 comparable 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.)