It would be unusual if it wasn't real in some cases. Fatigue is common following viral infections, in general. It's not that unusual to be left very weak for

It would be unusual if it wasn't real in some cases. Fatigue is common following viral infections, in general. It's not that unusual to be left very weak for months after a severe bout of influenza. The herpesviruses can do it too, as will most of the classics like measles, even a common cold if you're particularly unlucky. Just intuitively, a virus causes damage to the infected tissues. The more severe/long-lasting the infection before recovery, the more likely some of that damage is going to be permanent in the form of scar tissue, rather than normal healing. It almost certainly is associated with depression. Both in that if there is reduced lung function, etc. it may cause depression. But also in that depression can make an otherwise tolerable physical burden intolerable.