If you think that it is to wish speedy recovery you are wrong. It is actually too save the sneezer’s soul and preserve your own health!
Origins, The Justinian Plague
The Romans used to say “Jupiter preserve you” when in ancient time someone sneezed. It was a way to wish you health and call on the protection of the gods. The meaning morphed slightly under Pope Gregory. Pope Gregory the Great introduced the term “God Bless You” in the sixth century, 1,500 years ago! During the Justinian plague in the Eastern Roman Empire (541 AD), the Pope commanded that everyone who sneezed should be blessed immediately by saying “God bless you” and add the sign of the cross on their lips. This was ordered to call on Heavenly protection against the spread of the plague, to bless both you and the culprit. It was adopted so widely that only a century later it became a regular custom and people said it without thinking or thinking too much about the meaning of the phrase.
The Everyday Superstitious Meaning
There is also the superstition explanation which is a spin-off from the original. In fact, during the time of the plague it was believed that part of the soul was leaving the body while sneezing. The loss of part of your soul made you more vulnerable to temptation and to the devil. “God Bless You” took immediately the meaning of protecting your soul from your weakened state. It was also believed that the blessing would allow part of soul that left the body to return.
The Opposite was True at Times
Depending on the time and location, at times the sneeze was actually believed to be important as it was expelling the devil inside you. In that case, thanking after a bless you was considered dangerous as it might have invited the devil back.
So be careful to the times when you say bless you!