Flu-like symptoms after intense stress or exertion, when resting, can be an example of adrenaline activating the immune system to combat infection.

“And that's actually supported by something that you've probably experienced before, which is if you've ever worked, worked, worked, really hard or you've been a caretaker for somebody else or studying for exams and people around you are getting sick and you're just powering through it and you're not getting sick, but then you stop. And you turn in your final exam, you stop taking care of somebody else, or you finally stop and rest, or you go on vacation and then you get sick. Well, you've just experienced the effect that adrenaline, epinephrine, can have in activating your immune system by way of the nervous system in order to keep fighting and combating infection.”