Bless you, or God bless you, is a common English expression addressed to a person after they sneeze. The origin of the custom and its original purpose are unknown.
 Origins and legends
Several possible origins are commonly given. The practice of blessing a sneeze, dating as far back as at least 77 AD, however, is far older than most specific explanations can account for.
A legend holds that it was believed that the heart stops beating and the phrase "bless you" is meant to ensure the return of life or to encourage your heart to continue beating.
One explanation holds that people used to believe that your soul can be thrown from your body when you sneeze, that sneezing otherwise opened your body to invasion by the Devil or evil spirits, or that sneezing was your body's effort to force out an invading evil spirit. Thus, "bless you" or "God bless you" is used as a sort of shield against evil.
Another version says that the custom originally began as an actual blessing. Gregory I became Pope in 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.
A variant of the Pope Gregory I story places it with Pope Gregory VII, then tells the common story of "Ring Around the Rosey" being connected to the same plague.
Alternatively, it may be possible that the phrase began simply as a response for an event that was not well understood at the time.
Another belief is that people used to see sneezing as a sign that God would answer your prayers or an omen of good fortune or good luck. In this case, "Bless you" would be in recognition of that luck.
Tibetan Buddhists believe a sneeze (like meditation, falling asleep, preparing to die) can provide a moment of "clear consciousness," when people are opened to greater understanding.