No one knows for sure where the saying God bless you as a response to someone sneezing comes from. However, we do know the blessing is old and has many possible origins. We also know that the practice, as used when someone sneezes, is not in the Bible. Though, of course, declaring "God bless you" is in the Bible. It was used by the Israelites (Numbers 6:24) and by the early Christians as a benediction.
Why do we say 'God bless you' in response to a sneeze?
Likely blessing someone who sneezes originated in the first century from a superstition; people believed that sneezing was the body's attempt to get rid of evil spirits. Some believed that the heart stopped when a person sneezed and that a sneeze may make a person vulnerable to evil spirits. The phrase, God bless you, became a sort of protection or verbal talisman to protect the sneezer.
We see similar practices in other cultures. For example, the Spanish "Salud" (health), German "Gesundheit" (health), Gaelic "Sláinte" (good health), and Bengali "Jeebo" (stay alive) are all responses to sneezing.
During the Italian plague in AD 590, people thought sneezing signaled that a person was infected. Legend has it that Pope Gregory I declared that people should respond to a sneeze by saying "God bless you" then make the sign of the cross over their mouth.
Today, there is nothing wrong or unbiblical with invoking God's blessing on someone who sneezes (and on those who don't!).
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