The tradition of blessing someone after a sneeze is so old that even the Roman scholar Pliny the Elder wrote about it in Natural History (77 CE) and puzzled over its origins. While we don’t know exactly why people feel the need to bless someone after a sneeze, there are several theories.

Two common theories have to do with demons and spirits. It was once believed that the soul temporarily left the body during a sneeze, leaving it vulnerable to the clutches of Satan. The blessing was intended to keep the devil away from the sneezer’s soul. According to another theory, a person sneezed to expel evil spirits from their body, and the blessing was given to prevent them from going right back in.

Some theories relate to medical explanations. During the Renaissance era, it was believed a sneeze caused the heart to momentarily stop. The blessing was a brief prayer that the heart would not fail completely. Alternatively, it was once thought that sneezing was a sign of the plague. Because relying on medical science was often not an option then, only a blessing from God could save someone from their fate.

Yet another theory suggests that a sneeze was a sign that God was blessing someone with good fortune. The Bless you acknowledged the person’s fortune while thanking God and reinforcing a person’s good luck. One last theory posits that people thought the sneeze itself was a blessing that passed good fortune onto others, and people would politely return a Bless you as a show of good manners.