What Causes Chronic Fatigue Syndrome ?

The simple answer is that no one knows. There is no simple answer as to what triggers it, and no simple answer as to what has gone wrong in the body.

Onset

There seem to be a number of triggers, including viral illness and stress.

For me, it was the cumulative effect of virus after virus, recovering a little less after each one, as I followed the wisdom of the time of struggling on and on at work, until the situation became chronic, and CFS set in.

Some may try to connect it with SAD (Seasonal Affective Disorder), however, though some of the symptoms are the same, CFS sufferers are affected all year around.

Mechanisms

One of the more prominent thoughts is imbalance in the hormone system (adrenal, thyroid, etc) causing the immune system to be over activated, or, surprisingly, under activated. However, hormone levels for CFS patients are within the "normal" bands. The theory goes that a combination of low "normal" levels may be, in fact, not optimal.

There's also some evidence of muscle dysfunction where there is damage to the mitochondria.