We learn a number of social cues from an early age. It’s impolite to cough without covering our mouths. We say thank you when people give us things like money or cake. And when someone rears back and explodes in a violent expulsion of snot, we say bless you.

Why do we do this? What does a blessing have to do with sneezing? Did anyone ever believe a demon flew out of our noses as we honked one out?

Recorded civilization hasn’t done such a great job of tracking this peculiar ritual. Mentions of the bless you reaction date back to as early as 77 C.E., though no explanation is usually given. What is clear is that people tended to acknowledge sneezes as a sign of good health that prompted salutations. Greeks and Romans followed up a projection of mucus with phrases like live long and may Jupiter bless you.

That positive connotation changed with Pope Gregory I while Europe was in the throes of the bubonic plague, or Black Death, in the 6th century. Because sneezing was a symptom of illness, the Pope thought it would be proper to say God bless you as a little extra insurance from what was otherwise near-certain death.

There was also a pervasive myth that the heart would briefly stop while sneezing, likely due to changes in blood flow that might cause a brief delay between heartbeats. People may have said bless you to make sure the heart would continue beating rather than stop altogether, or as a form of congratulations: Bless you, Carl. That sneeze didn’t kill you.

Cultures who believed spirits could either be ejected or evil spirits transmitted during a sneeze may have also adopted the phrase to help ward off such exchanges.

However it came about, it’s clear we’ve adopted a blanket policy when it comes to sneezing. When people don’t say bless you, we begin to suspect they don’t care about our well-being. As etiquette columnist Miss Manners once observed, it’s considered more rude for people getting hit with snot shrapnel to bypass the bless you than it is for the person detonating the germ bombs to fail to say excuse me. Leave it to a plague to make a lasting impression on people.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.