According to casual scroll of Wikipedia, the reply of “God bless you” as a response to sneezing has (as far as I can tell) some conflicting beginnings. Some claim it formally originated during the plague, as it was a first sign of falling ill. Some claim it was due to the belief that your soul could be thrown from your body during a sneeze, opening you up to evil spirits, or that sneezing was a body’s response to force out an invading spirit. Some thought your heart stopped when you sneeze and that would encourage it to pick back up.

They also provide a pretty broad list of responses across multiple languages (https://en.wikipedia.org/wiki/Response_to_sneezing). My favorite is the Serbian one used mostly with children, which apparently translates to “go away kitten”.