Fatigue is a very vague term and it may be caused by almost any chronic diseases.
Common causes are-
A) Anemia
B) Malnutrition
C) Obesity
D) Sleep disorders
E) During and in recovery period in case of any infection
F) Chronic infection like HIV, hepatitis B,C, TB etc
G) Hormonal disturbances- thyroid , hypercalcaemia, electrolyte disturbances like less sodium , diabetes.
H) Malignancy
I) diseases of lungs, heart, liver ,kidney, brain etc
For treating fatigue , first the underlying causes should be diagnosed and treatment should be directed towards the underlying causes.
In most of the cases, if there is no history of any disease , fatigue is due to Anemia, malnutrition, decreases fluid intake, stress.