Normal people experience occasional bouts of fatigue but those episodes are transient and temporary and do not last longer than an afternoon or an evening and disappear after a good night's sleep. Fatigue occurs in many diseases.
Patients use the term fatigue to describe malaise associated with infections.
Fatigue may also indicate muscle weakness and tiredness and limited exercise tolerance. Fatigue is a common symptom of influenza or flu-like syndrome. Fatigue is a frequent symptom of heart failure. Anxiety, anemia and depressions are frequent sources of fatigue. Overactive thyroid and underactive adrenal glands are too often misdiagnosed causes of fatique.