Before Covid started, when a person had a particularly bad flu, it sometimes happened that they'd feel like crap and weak for a good long while after recovering: sometimes for weeks and months.

I'm not completely certain about what "long-covid" is, but this phenomenon could be a combination of this known effect as well as the after effects of the inhuman living conditions society is forcing on people: unnatural social isolation, constant media induced fear and stress, lack of sunshine and exercise, bad diets (because there's fewer fun entertainment options), etc.