Almost all cultures have some ritually offered words for someone who sneezes -- the ancient Greeks, the American Indians, the rabbinical Jews, Asians, etc. Many of these groups (Talmudic Jews included) believed that sneezes could be mortal, that the soul could leave the body through the mouth during a sneeze.
It is Pope St. Gregory the Great, however, whom we have to thank for the practice of saying "God bless you" after someone sneezes. He was Pope during a time when Rome was ravaged by a certain pestilence, the symptoms of which were sneezing and yawning. In A.D. 600, he decreed that when someone sneezes, one should respond with "God bless you" to ask God's blessings for the person's health, and that when a person yawns, the Sign of the Cross should be made on his mouth.
Quoted from Fisheaters
I love the custom of blessing people when they sneeze. It's a courteous way of showing your concern for their health, and an easy way to sneak in a little prayer for them, to boot. I think everyone still does this, don't they? But, I totally see how Pope St. Gregory's other recommendation didn't catch on...
Imagine. How would it work exactly? Would all your friends and family members rush to see who can get to you first to sign you with the cross when you yawn? Maybe that would just be the job of the person standing closest to the yawner. Or does everyone stand in line and bless your mouth, one by one? Do you step over to the man on the bus and make the Sign of the Cross over his mouth? What do you do if you hear me yawning when I'm talking to you on the phone, bless the receiver? I can see accidents and mishaps and people getting smacked in the face... And I don't know about you, but my kids would just get sick of it, I yawn so much.
In fact, I'm yawning right now. Oh, no! Now I've started something, haven't I? Or is yawning contagious online?
Here. I'll bless the monitor, just in case.
Anyone with a cold out there?
Well, God Bless You!
And those of you who are neither yawning nor sneezing?
God Bless You, too!