implying that genetic factors may increase the risk of susceptibility to the illness. 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. One review found higher Epstein–Barr virus (EBV) antibody activity in patients with CFS, and that a subset were likely to have increased EBV activity compared to controls. Viral infection is a significant risk factor for CFS, with one study finding 22% of people with EBV experience fatigue six months later, and 9% having strictly defined CFS. A systematic review found that fatigue severity was the main predictor of prognosis in CFS, and did not identify psychological factors linked to prognosis. A review of long COVID research found half of those affected met diagnostic criteria for ME/CFS. Another review found that risk factors for developing CFS after mononucleosis, dengue fever, or Q-fever included longer bed-rest during the illness, poorer pre-illness physical fitness, attributing symptoms to physical illness, belief that a long recovery time is needed, as well as pre-infection distress and fatigue. The same review found biological factors such as CD4 and CD8 activation and liver inflammation are predictors of sub-acute fatigue but not CFS. A study comparing diagnostic labels found that people labelled with ME had the worst prognosis, while those with PVFS had the best. Whether this is due to those with more severe or longer-lasting symptoms results in a label with the description of ME, or if being labelled with ME adversely causes a more severe or prolonged illness is unclear. Pathophysiology Neurological A range of neurological structural and functional abnormalities is found in people with CFS, including lowered metabolism at the brain stem and reduced blood flow to areas of the brain; these differences are consistent with neurological illness, but not depression or psychological illness. The World Health Organization classes chronic fatigue syndrome as a central nervous system disease. Some neuroimaging studies have observed prefrontal and brainstem hypometabolism; however, sample size was limited. Neuroimaging studies in persons with CFS have identified changes in brain structure and correlations with various symptoms. Results were not consistent across the neuroimaging brain structure studies, and more research is needed to resolve the discrepancies found between the disparate studies. Tentative evidence suggests a relationship between autonomic nervous system dysfunction and diseases such as CFS, fibromyalgia, irritable bowel syndrome, and interstitial cystitis. However, it is unknown