system
Given the following post, respond as if you're commenting on Reddit:
human
Where does the pain come from when you have a viral infection (like covid or the flu)?

I am currently under a viral infection and I honestly feel like I just have been hit by a train... twice. 
So I was wondering who/what is activating all these pain receptors in my head and my muscles. Is it the virus even thought it's entry way was the lungs? Is it just a side product of inflammation? But other infections don't lead to this.
Thanks!
gpt
It's a side effect of widespread inflammation. Yes, even if a virus has a tendency to replicate in a particular cell type (such as those in the respiratory tract), if it initiates enough of a local inflammatory response (in terms of magnitude as well as duration) , it will get amplified to a systemic level with the goal of recruiting other non-resident immune cells and shift resources to fighting the infection. This systemic inflammation is mediated and induces the elevation of all sorts of cytokines in the plasma, some of which (e.g. TNF) are known to induce muscle breakdown (proposed teleological reason is the increase in amino acids in the blood, feeding to the liver so it can make more opsonins and other immune system components), while others induce pathways that also overlap with pain receptor activation (e.g. Prostaglandin E2), and also of course.. There are more anatomically mediated changes that cause pain (e.g. Vascular tone near the brain, congested sinuses and the pressure they exert, etc). Other infections may not induce such symptoms possibly because they don't evoke a strong enough systemic inflammation like more stealthy viral infections or those that replicate like there's no tomorrow.