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Bless you, or God bless you, is a common English expression typically addressed to a person after they sneeze. Alternative responses to sneezing adopted by English speakers are the German word gesundheit (meaning "health"), the Irish word sláinte (meaning "good health"), and the Spanish salud. The origin of this custom and its original purpose are unknown.
Origins and legends 
Several possible origins are commonly given. 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.
One explanation holds that the custom originally began as an actual blessing. 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.
Another explanation suggests that people used to believe 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, "bless you" or "God 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.
Alternatively, it may be possible that the phrase began simply as a response for an event that was not well understood at the time.
See also 
- Snopes Urban Legends - Bless You!
- Straight Dope. Ed Zotti, Editor. Why Do We Say "God Bless You" After a Sneeze?, Straight Dope, 27 September 2001.
- Madsci.org, Mad Scientist posting by Tom Wilson, M.D./PhD, Pathology, Div. of Molecular Oncology, Washington University School of Medicine
- Gutenberg.org, Project Gutenberg story by T. Crofton Croker, 1898.
- Re: Why does plucking my eyebrows make me sneeze?, MadSci Network posting by Robert West, Post-doc/Fellow, 1997-08-05