Gesundheit, as you may know, is simply German for "health." I always thought it a little odd that it was such a long word for something that's so short in English, and even odder that it would bear no obvious relation to the English word. Its relation to English becomes a little clearer if you know just what English words it's related to.
Gesund is cognate with sound (as in "safe and sound" or "of sound mind"), and it means "whole" or "healthy." The English word lost the ge- prefix during Middlel English. The suffix -heit is cognate with the English -hood, which means essentially "condition or quality."
So if you were to form an English calque (piece-by-piece literal translation) from gesundheit, you would get soundhood (though English has soundness instead).