From the net:
Wishing someone well after they sneeze probably originated thousands of years ago. The Romans would say “Jupiter preserve you” or “Salve,” which meant “good health to you,” and the Greeks would wish each other “long life.” The phrase “God bless you” is attributed to Pope Gregory the Great, who uttered it in the sixth century during a bubonic plague epidemic (sneezing is an obvious symptom of one form of the plague).
For the most part, the various sneeze responses originated from ancient superstitions. 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.
