The practice of blessing someone who sneezes, dating as far back as at least AD 77, however, is far older than most specific explanations can account for. Gregory I became Pope in AD 590 as an outbreak of the bubonic plague was reaching Rome. In hopes of fighting off the disease, he ordered unending prayer and parades of chanters through the streets. At the time, sneezing was thought to be an early symptom of the plague. The blessing (“God bless you!”) became a common effort to halt the disease.
Some have offered an explanation suggesting that people once held the folk belief that a person’s soul could be thrown from their body when they sneezed, that sneezing otherwise opened the body to invasion by the Devil or evil spirits, or that sneezing was the body’s effort to force out an invading evil presence. In these cases, “God bless you” or “bless you” is used as a sort of shield against evil. The Irish Folk story “Master and Man” by Thomas Crofton Croker, collected by William Butler Yeats, describes this variation. Moreover, in the past some people may have thought that the heart stops beating during a sneeze, and that the phrase “God bless you” encourages the heart to continue beating.
In some cultures, sneezing is seen as a sign of good fortune or God’s beneficence. As such, alternative responses to sneezing are the German word Gesundheit (meaning “health”) sometimes adopted by English speakers, the Irish word sláinte (meaning “good health”), the Spanish salud (also meaning “health”) and the Hebrew laBri’ut (colloquial) or liVriut (classic) (both spelt: “לבריאות”) (meaning “to health”).