Hypothyroidism, anemia, coeliac disease (that can occur without gastrointestinal symptoms), diabetes and certain psychiatric disorders are a few of the diseases that must be ruled out if the patient presents with appropriate symptoms. Other diseases, listed by the Centers for Disease Control and Prevention, include infectious diseases (such as Epstein–Barr virus, influenza, HIV infection, tuberculosis, Lyme disease), neuroendocrine diseases (such as thyroiditis, Addisons disease, adrenal insufficiency, Cushings disease), hematologic diseases (such as occult malignancy, lymphoma), rheumatologic diseases (such as fibromyalgia, polymyalgia rheumatica, Sjögrens syndrome, lupus, giant-cell arteritis, polymyositis, dermatomyositis), psychiatric diseases (such as bipolar disorder, schizophrenia, delusional disorders, dementia, anorexia/bulimia nervosa), neuropsychologic diseases (such as obstructive sleep apnea, parkinsonism, multiple sclerosis), and others (such as nasal obstruction from allergies, sinusitis, anatomic obstruction, autoimmune diseases, cancer, chronic hepatitis, some chronic illness, alcohol or other substance abuse, pharmacologic side effects, heavy metal exposure and toxicity, marked body weight fluctuation). Ehlers–Danlos syndromes (EDS) may also have similar symptoms. Medications can also cause side effects that mimic symptoms of CFS.Persons with fibromyalgia (FM, or fibromyalgia syndrome, FMS), like those with CFS, have muscle pain, severe fatigue and sleep disturbances. The presence of allodynia (abnormal pain responses to mild stimulation) and of extensive tender points in specific locations differentiates FM from CFS, although the two diseases often co-occur.Depressive symptoms, if seen in CFS, may be differentially diagnosed from primary depression by the absence of anhedonia, decreased motivation, and guilt; and the presence of somatic symptoms such as sore throat, swollen lymph nodes, and exercise intolerance with post exertional exacerbation of symptoms.
A 2009 meta-analysis showed that African Americans and Native Americans have a higher risk of CFS, though it specifically excluded other more common ethnicities worldwide, and it acknowledged that studies and data were limited.More women than men get CFS. A large 2020 meta-analysis estimated that between 1.5 and 2.0 times more cases are women. The review acknowledged that different case definitions and diagnostic methods within datasets yielded a wide range of prevalence rates. The CDC estimates CFS occurs up to four times more often in women than in men. The illness can occur at any age, but has the highest prevalence in persons between the ages of 40 and 60. CFS is less prevalent among children and adolescents than among adults.Blood relatives of those who have CFS appear to be more predisposed, implying that genetic factors may increase the risk of susceptibility to the illness.According to the CDC, "CFS is a biological illness, not a psychologic disorder", and those affected "are neither malingering nor seeking secondary gain". The World Health Organization (WHO) classifies CFS as a neurological disease in the ICD-11 for Mortality and Morbidity Statistics (ICD-11). Viral and other infections
Post-viral fatigue syndrome (PVFS) or post-viral syndrome describes a type of chronic fatigue syndrome that occurs following a viral infection. A recent review found Epstein–Barr virus (EBV) antibody activity to be higher in patients with CFS, and that a subset of patients with CFS were likely to have increased EBV activity compared to controls.