[Gibbon1]

I'm on the edge of what I know here but. I think the flu is innately self limiting because the virus only infects cells in the respiratory system, and generally only the upper part. It tends to kill people who are frail.

Covid19 most of the time mimics the flu. But because the receptors it uses are wide spread in the body it can an does cause systemic illness. In particular I see more and more comments from doctors about clotting and vascular issues.

Consider strep. You have step throat. And then you have scarlet fever. Covid19 is similar, presents at a mild respiratory disease mostly, or as a serious one with pnumonia, or deadly systemic.