My guess is many different places, depending on the patient.
I also wonder how many different cell types in the body have a faulty energy metabolism. Professor Olav Mella says that lymphocytes (B-cells, T-cells and NK cells) are especially effected by this faulty energy metabolism.
However, I'd like to know if all the different cell types in the body are affected, or is it just certain types of cells that have a faulty energy metabolism?
If it is just certain cell types, and those cell types turn out to be virally infected, then the presence of the virus in the cell might be able to explain the cellular energy fault.
But if
all (or most of) the cell types in the body have this faulty energy metabolism, well, there are no viruses that can infect every cell type (a given virus is only able to infect a small subset of cells types); so then the explanation for the faulty energy metabolism couldn't be a result of an infection of the cells, but would have to be caused by some global factor acting on all cells in the body, perhaps for example some specific autoantibody that targets the mitochondria.