It is unlikely that a singular cause of chronic fatigue will ever be found by science because there are always multiple physical and physiological factors involved and all must be systematically addressed for recovery to occur. Recovery can be slow more a marathon than a sprint. Patience, dedication and understanding are required.

Commonly the following factors are involved in chronic fatigue:

Nutritional deficiencies/imbalances such as protein, iron, zinc
or magnesium.

Gut and liver dysfunction which can be caused by food allergies
and gluten intolerance.

Heavy metal toxicity- particularly mercury, lead and arsenic.
 
Chronic infections e.g. mycoplasma, chlamydia and rickettsiae
specialised testing is involved in identifying these infections
because they are intracellular organisms.
Appropriate identification and treatment of these organisms
results in a dramatic improvement in fatigue in the majority.

Hormone imbalances – hormone systems possibly involved
include the adrenals, thyroid and the sex hormones.

All of the above factors must be addressed for a successful outcome. Chronic infections are almost always associated with an elevated body burden of heavy metals and both these conditions need to be treated.