I count myself extremely fortunate, as [knock on wood] I rarely get the flu. About once every 4 years on average, and most times when I get it, I'm through it in about 48-72 hours. And my clients are always very understanding and I'm able to still deliver while WFH, except in the very worst depths of about 12-24 hours.
There has got to be some genetic component where others have it way worse than I do, and yet still others skate by even better than I do. We know far less about biology than we think we do.