Cuz you blackout and your heart stops briefly when you sneeze

www.wisegeek.com/why-do-...

in the middle ages, it was believed that demons were wrestling with angels in the seat of your soul (your liver or heart)...

when you sneezed, it was believed the demon was ousted. hence you were blessed by god that you survived.

truth is your heart does stop momentarily when you sneeze, so it is a blessing that it restarts, hence a bless.

gazunheit

gesund = health heit = ness.....gesundheit basically means you wish somebody healthiness. (But most people think it means God Bless You.)