Where does bless you originate from?

Answer 1

Two theories, take your pick;

1). An ancient belief was that your spirit was ejected from your body when you sneezed and someone nearby would bless you so that Satan or his demons wouldn't be able to inhabit the stolen body, until the rightful spirit returned.

2). Sneezing was one of the symptoms of the Bubonic Plague and people around you would try to invoke divine power to deflect the rest of the plague from you.