system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people often get sick after pushing themselves?

I have noticed that oftentimes after prolonged physical and/or mental stress there is a tendency that people get sick. Is there a pattern here and if so, why does the body react this way?
gpt
When you're under stress, your body releases the hormone cortisol, which helps in the fight-or-flight response. It readies your body to be amped up to react in stressful situations - but if you don't react accordingly, the cortisol builds up with nowhere to go.

If you keep elevating the levels of cortisol without using them, it throws your body out of balance and can adversely affect bone density, memory, weight gain, healthy heart function, cholesterol levels, and so much more.

Think of it like a dehumidifier in a room. Its purpose is to collect water from the air and collect it in a bucket-like thing. If you empty it regularly, it can keep performing like it's supposed to. But if you forget about it for a while, it keeps collecting water and will eventually spill over. 

The body favours being in a state of homeostasis; it's also remarkable in that you can throw a lot of crap its way and it'll adjust. But homeostasis is where it's happiest and keeping it in an extreme state for too long without relief will cause it to "voice its displeasure" with you and it'll show that displeasure physically.