God bless you (variants include God bless or bless you) is a common English expression, used to wish a person blessings in various situations, especially as a response to a sneeze, and also, when parting or writing a valediction.
The phrase has been used in the Hebrew Bible by Jews (cf. Numbers 6:24), and by Christians, since the time of the early Church as a benediction, as well as a means of bidding a person Godspeed. Many clergy, when blessing their congregants individually or as a group, use the phrase "God bless you".
A typical polite response after being told "bless you" in response to sneezing is to thank the person who has said it.
God bless you Wikipedia
National Geographic reports that during the plague of AD 590, "Pope Gregory I ordered unceasing prayer for divine intercession. Part of his command was that anyone sneezing be blessed immediately ("God bless you"), since sneezing was often the first sign that someone was falling ill with the plague." By AD 750, it became customary to say "God bless you" as a response to one sneezing.
The practice of blessing someone who sneezes dates as far back as at least AD 77, although it is far older than most specific explanations can account for. 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 spelled: "לבריאות") (meaning "to health").
In Persian culture, sneezing sometimes is called "sabr =صبر," meaning "to wait or be patient." And when trying to do something or go somewhere and suddenly sneezing, one should stop or sit for a few minutes and then re-start. By this act the "bad thing" passes and one will be saved. This is observed in Indian culture as well.