Fatigue may be a symptom of almost any medical condition. For the purpose of this topic, the differentials discussed concentrate on people presenting with fatigue or where fatigue is the only symptom. Conditions in which fatigue may not necessarily be an initial complaint, but is still regarded as a markedly significant and debilitating symptom, are also included.
There are numerous definitions and classifications of fatigue, reflecting the multitude of interpretations, depending on being a patient, a physician, a biologist, or a physiologist. A common and practical definition defined fatigue as a sensation of exhaustion during or after usual activities, or a feeling of inadequate energy to begin these activities. 
Primary-care-based surveys have shown that between 11% and 33% of patients report significant fatigue, resulting in approximately 7 million office visits per year in the US. Fatigue is also a common complaint in the general population, with a prevalence between 4.3% and 13.4%.     
In the primary care setting, a medical or psychiatric diagnosis is found in the majority of patients presenting with recent fatigue (at least two-thirds).      A Dutch study, assessing the final diagnosis in patients presenting to general practitioners with a variety of complaints, identified a specific diagnosis in 63% of those complaining of general weakness or tiredness for any length of time.  The most common diagnoses were, by descending order, viral illness, upper respiratory infection, iron-deficiency anemia, acute bronchitis, adverse effects of a medical agent in the proper dose, and depression or other mental disorder.  The most frequent psychiatric illnesses included major depression, panic disorder, and somatization disorder.
Fatigue can be divided into categories based on origin, attribution, and duration of symptoms. The origin of fatigue may be central (brain-derived) or peripheral (usually a neuromuscular origin). It may be attributed to physical illness; psychological (e.g., psychiatric disorder), social (e.g., family problems), and physiological factors (e.g., old age); or occupational illness (e.g., workplace stress).  The duration of symptoms may refer to recent fatigue (symptoms lasting <1 month), prolonged fatigue (symptoms lasting >1 month), and chronic fatigue (symptoms lasting >6 months).
When unexplained, clinically evaluated chronic fatigue can be separated into chronic fatigue syndrome and idiopathic chronic fatigue.   Chronic fatigue syndrome represents a small subset of those who complain of actual chronic fatigue. Even in patients with fatigue of 6 months or longer in duration, the prevalence is <40%. European studies have shown that patients with fatigue lasting longer than 6 months were given a diagnosis of chronic fatigue syndrome in one third or less of cases.