system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What stops me from being reinfected with an illness if I stay in the house where I was sick?

Wouldn't I just be breathing the same infected air?
gpt
It can happen, but by and large once you’re infected with something, your immune system learns how to deal with it. It begins to make antibodies that are like medicine customized for the exact thing that infected you. 

So if you get infected a few days after feeling better, the germs are killed almost instantly upon entering your body and the infection never takes hold.