> When you feel sick don't stay in bed, go for walks, a large part of your immune system is passive and rely on muscle contraction to work optimally.

Do you have a source for this? I haven't heard this before and genuinely want to learn more about it. A cursory Google search didn't find anything related to this

I have doubts about that. The feeling of sickness is basically our body telling us not to move. Sick animals also don't move much.

And a common reaction to being sick is shivering, so maybe the body is already compensating for the lack of movement.