Fatigue is not a specific symptom and can be caused by many diseases. It is common to feel tired and weak after an illness, and different diseases cause different levels of fatigue.
Physiological factors such as lack of sleep, lack of nutritional supplements and excessive exercise can cause fatigue, chronic fatigue syndrome, endocrine system diseases such as diabetes, hypothyroidism and adrenal insufficiency can cause fatigue, psychological factors such as depression and neurasthenia can cause fatigue, as can wasting diseases such as various types of anaemia, tuberculosis and malignancies.