pseudoparanormal

Many people say "God bless you" after someone sneezes as a polite gesture.  What a lot of people don't know is why we say that. During the time of the plague, Pope Gregory the Great would say it to people that were sick because it was believed that the soul left the body when you sneezed and you were welcoming the person back to life by saying, "God bless you." Another common misconception is that when you sneeze, your heart stops, so by saying "God bless you," to people it would protect them from death. In some cultures, it was thought that sneezing was a way to force evil spirits that were trying to enter the body away. The blessing was a way to protect people around them.

http://www.loc.gov/rr/scitech/mysteries/sneeze.html