system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why it's a cultural norm to say something like "bless you" or "gesundheit" after a sneeze?
gpt
Way back in the middle ages, when germ theory was centuries away, all people knew was that when people were sick, they would sneeze and/or cough. And if people got sick enough, they would die.

There were lots of crazy theories why. For instance, some Italians thought that the bad air (Italian: Mal Aria, where we get "malaria" from) of a swamp would make you sick, so they would bring you into a clean room, and fill it with purfumes. Being in a clean room with clean food and clean water usually meant the healthier folk got better on their own.

Other people thought that it was evil spirits, tainting the blood and making you ill.

Either way, they knew getting sick was bad news, because treatments, even the effective ones, often resulted in previously healthy people dying.

So, in this atmosphere, if you heard somebody sneeze, if you cared about them (And even if you didn't, really, since outbreaks could and did spread pretty easily) you would bless them in hopes god would prevent them from getting sick, or wish them "gesundheit" (German, for  "good health"), or any of the many other linguistic responses wishing a person to not fall ill.