Man first believed that a person's soul resided in the head and that a sneeze was actually allowing some of your life to escape.
You know how unsanitary things were back then, so with all the sneezing and then the dying, this just reinforced the belief.
With the teachings of Aristotle & Hippocrates in the 4th century, people began to understand that sneezing was actually the body's way of expelling foreign matter that had gotten into the nostrils.
The Romans then decided that a sneeze was the body's attempt to exorcise a demon or sinister spirit.
When someone sneezed, the Romans said something like "Congratulations" or "Good luck to you".
The actual Christian version of "God Bless You" began in the 6th Century by Pope Gregory. At that time, a plague was raging through Italy with the first symptom being a lot of sneezing. The plague acted so quickly, that very soon after someone started sneezing, they died.
Pope Gregory began appealing to the healthy people to pray for the sick. He insisted that phrases like "May you enjoy good health" be replaced with "God Bless You!"
If no one was around, the sneezer had to say "God help me!"
With so much death being associated with sneezing, this is also where the phrase "Not to be sneezed at" began.
Sneezing ain't no joke.
For the good of your fellow humans, please cover your mouth and nose when sneezing.
As an interesting experiment, every time I hear someone sneeze this coming week, I'm going to say "Congratulations! You just exorcised a demon. Praise the Baby Jesus!" (with a serious Southern accent, of course).
I just want to see what they'll do.