On this day in 600 A.D., Pope Gregory I (also known as “Gregory the Great”) issued a papal decree requiring all Christians to use the expression “God bless you” when in the presence of someone who sneezed.

At the time the decree was issued the bubonic plague was spreading across Europe.

During this period the plague killed between 25 and 50 million people.

It was believed that sneezing spread the disease and in order to protect themselves from the illness Christians would invoke God’s blessing.

Some also believed that the act of sneezing left the body unguarded for a moment and this could be enough time for the Devil to enter a person’s soul.

Requesting God’s blessing at this moment of vulnerability would protect the person who sneezed from such a fate.