system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some bacteria normal in some parts of the body, but become dangerous when moved to other parts?
gpt
The digestive system and skin have evolved to interface with the outside world, and the outside world contains a bunch of pathogens, so they're quite resistant to disease.  They renew cells at a high rate, and produce acids and enzymes that keep bacterial growth under control, among other countermeasures.

Most internal organs are more vulnerable than that, because they only interface with other internal parts of the body that the skin and digestive system have already filtered harmful stuff out of.  For this reason, they haven't evolved to be resistant in the same way.

An analogy:  Putting gut bacteria in the brain is like delivering a big pile of explosives to the parking lot of the head office of a large chemical supply company.  They'd be in trouble.  They don't have the knowledge, equipment, or facilities to safely handle and store explosives.  This stuff belongs in a secure warehouse, not in the office parking lot!  It would cause chaos.  They'd have to evacuate the building, call security so nobody steals the explosives, and find experts to transport them safely to somewhere with people who can handle them.  The head office has evolved to be in contact by telephone and internet with the warehouse that handles explosives, not to directly handle explosives themselves.