Those of you with fatigue might want to get tested for the MTHFR gene mutations, if you haven't already. Our 16 year old son was fatigued for well over a year after having mono, and I was wondering if he was going to end up with CFS but then we had him tested for the MTHFR gene mutations and it turns out that even though he only has one copy of the C677T mutation he wasn't able to process enough folate for his body's needs when he had mono and it pulled him into a downward spiral. Taking methyl folate (the activated form of folate that people with this gene mutation have trouble making the enzyme for) has helped him tremendously. Or if you don't get tested you can just try taking the methyl folate for a couple of months. There are two easy to find brands, but you should probably try the one that has the 5 mg dose, that's what my son is taking.
He noticed a result within a few hours of taking the 5 mg dose, but this was after taking 1 mg a day for a couple of weeks. Over the next few months he got less and less tired which I assume was because he must have been anemic from not having enough methylfolate. I think that a lot of people with chronic fatigue probably also have gut dysbiosis and maybe food allergies that are contributing to their problems. I don't think it is usually caused by one thing. We are also still working on getting rid of candida (he tested positive) but the candida die off just makes him depressed and apathetic, he isn't really tired now very much.
I forgot to mention that he was already on the GAPS diet when he started the methylfolate. Then in January I thought he was doing OK, so I started letting him have potatoes and my home made sourdough bread, which does not have any sweetener in it. Boy, was that a mistake. When he went back on GAPS he had a lot of die off, and we are back to square one on the candida issue.