Some Catholics attribute to St. Gregory the use of the benediction “God bless you,” after sneezing, and say that he enjoined its use during a pestilence in which sneezing was a mortal symptom, and was therefore called the death-sneeze. Aristotle mentions a similar custom among the Greeks; and Thucydides tells us that sneezing was a crisis symptom of the great Athenian plague. The Romans followed the same custom, and their usual exclamation was “Absit omen!” We also find it prevalent in the New World among the native Indian tribes, in Sennaar, Monomatapa, etc. etc.
It is almost incredible how ancient and how widely diffused is the notion that sneezing is an omen which requires to be averted. The notion prevailed not only in ancient Greece and Rome, but is existent in Persia, India, and even Africa. The rabbins tell us that Jacob in his flight gave a sneeze, the evil effects of which were averted by prayer.
In the conquest of Florida, when the Spaniards arrived, the Cazique, we are told, sneezed, and all the court lifted up their hands and implored the sun to avert the evil omen.
In the rebellion of Monomatapa, in Africa, the king sneezed, and a signal of the fact being given, all the faithful subjects instantly made vows and offerings for his safety. The same is said respecting Sennaar, in Nubia, in Sweden, etc.
The Sadder (one of the sacred books of the Parsees) enjoins that all people should have recourse to prayer if a person sneezes, because sneezing is a proof that the “Evil Spirit is abroad.”
Foote, in his farce of Dr. Last in His Chariot, makes one of the consulting doctors ask why when a person sneezes, all the company bows? and the answer given was that “sneezing is a mortal symptom which once depopulated Athens.”
“In Sweden, ... you sneeze, and they cry God bless you.” —Longfellow.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894