Why do we say, "Bless You!" when someone sneezes? Do all people say it?

National Geographic reports that during the plague of AD 590, "Pope Gregory I ordered unceasing prayer for divine intercession. Part of his command was that anyone sneezing be blessed immediately ("God bless you"), since sneezing was often the first sign that someone was falling ill with the plague." By AD 750, it became customary to say "God bless you" as a response to one sneezing

from Wikipedia