system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5- (How) does bacteria suddenly appear in an environment that is suited to it?

Where was it before? How was it surviving before the environment became appropriate for it? For example- anaerobic bacteria multiplying in a dead body once blood stops pumping, but where was it before?
gpt
It was already there.  Bacteria are everywhere.  They're all over you.  They're in you.  They're in the air.  They're in the soil.  When you're dead and your immune system thus fails the bacteria can now go to town on you.