What is Chronic Fatigue?

This past week I have been totally out of it. I stay tired all of the time, but this has not been one of the better weeks.
One of the things I have sought an answer for is, why is fatigue so common?
Whenever someone gets sick, fatigue is one of the first symptoms. Why?

I don't have an answer for this one. My assumption is we feel fatigued from the body working overtime to fight infection. It seems to me fatigue should not be that hard to reverse. If we consume to the needed nutrients to support the organs, vitamin C for the adrenals, L-Glutamine for the stomach, probiotics for the intestines, and omega 3's for the heart and brain, the body should rebuild itself. Right? Obviously not, something is missing.

I have read through many threads on many forums, there are some people who practically supplement every nutrient the body could possibly need. Yet they still feel fatigued. Supporting the methylation process, the GAPS diet to rebuild the gut, and detoxification can all help...but its not all that common to for someone to say "I found that magic bullet that cured me". Don't get me wrong, all of these methods can be invaluable. The body needs all of these nutrients, sick or not.

     In my opinion, it looks almost like the body just doesn't use the food, supplements, medications, etc. that we provide well. Like the building blocks are there, the body just doesn't (or cant?) use them. I wonder why that is?

     Healing the stomach and intestines can certainly improve digestion and absorption. This can make a big difference for someone who has had
high doses of long-term antibiotics. Antibiotics kill the beneficial bacteria in the gut. These bacteria allow us to break down food sufficiently to be digested. Food that is not broken down enough cannot be absorbed and utilized. This would cause anorexia like symptoms even though the person is eating plenty.

     The fatigue, however, does not start after lyme treatment. The treatment can make it worse, but its not the cause. Fatigue is usually one of the first symptoms to appear, it was for me (along with insomnia.) but why?

What is there to curb this fatigue? I am still searching...