[garganzol]

>How can a few days of viral infection deplete NAD for months?

That's a mystery. A longer than usual disease progression fully depletes the NAD, while a flu does this partially? A more involved attack on mitochondria?

On a general note, metabolism is like a self-supported recursive function. But when one stack frame collapses, the whole calculation breaks like a house of cards causing the interlocked dependencies of pathological phenotypes.

>Also shouldnt tissue hypoxia be quite evident?

Not at all. For example, not all the labs can test for lactic acidosis. And even the ongoing lactic acidosis may be so localized that the measurement is negligible up to the error of measurement.