There are several theories for this. One theory says that the phrase God Bless you came from an Islamic practice which originated over 1400 years ago. According to the Islam religion, when someone sneezed, he should say All Praises to God and in response the people would reply May God Bless You. Another explanation is that when people sneezed it was believed that the heart would miss a beat; in that microsecond, the skip would allow the devil to enter the body. Therefore, saying God bless you would stop the devil from entering one's body uninvited. Some people believed that a sneeze causes the soul to escape the body through the nose. Saying bless you would stop the devil from claiming the person's freed soul. Others believed the opposite: that evil spirits use the sneeze as an opportunity to enter a person's body. There was also the misconception that the heart momentarily stops during a sneeze (it doesn't), and that saying bless you was a way of welcoming the person back to life. We now know that sneezing is a reflex action and is most often the sign of something relatively benign, such as a cold or allergy. If you've just sneezed, Rob, I think I'd rather give you a box of Kleenex. The custom of saying God bless you after a sneeze was begun literally as a blessing. Pope Gregory the Great (540-604 AD) ascended to the Papacy just in time for the start of the plague (his successor succumbed to it). Saying God bless you when you sneeze keeps the devil from flying down your throat. When someone sneezes, say God bless you and may the devil miss you.. When you sneeze your soul tries to escape and saying God bless you crams it back in (said by Millhouse in an episode of The Simpsons).
http://www.answers.com/Q/Why_do_people_say_'God_Bless_You'_when_you_sneeze http://people.howstuffworks.com/sneezing.htm http://www.straightdope.com/columns/read/1958/why-do-we-say-god-bless-you-after-a-sneeze http://www.straightdope.com/columns/read/1958/why-do-we-say-god-bless-you-after-a-sneeze
why do you say bless you when a person sneezes