system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is eating food off the ground in any way beneficial to your immune system?

I'm sure we all know the type: someone drops part of their sandwich on the ground, picks it up, eats it, then claims that they are, "strengthening their immune system" by deliberately exposing themselves to whatever was on the floor.

Is there any truth to this? It seems to logically follow that giving your immune system targets to work on will keep it 'up tondate' and active. But do people who do this get sick less than people who don't?
gpt
your immune system is like the borg.  it is very adaptable.  once it encounters one enemy, it'll be able to adapt to it and fight it off next time (assuming, that you survived the first encounter).  

being exposed to germs, bacteria, and viruses invoke your immune system.  once your immune system is familiar with a certain pathogen, it can fight it.  but your immune system needs to survive the exposure.  there is a "general strength" of your immune system, the more pathogens it's familiar with, the stronger it's overall response (ie a higher white blood cell count, which is basically your soldiers)

so someone who has lived in a very dirty environment will likely be much more able to fight off a new pathogen vs someone who's immune system has never been invoked (ie living in a clean room.).

one way to artifically strengthen your immune system is by vaccines, which are basically the pathogen but in a much weakened state so the average immune system will have no problem fighting it.