BDNF and myalgic encephalomeyelitis/chronic fatigue syndrome: Meanwhile the pathological mechanism of CFS is still uncertain, major findings in etiology of recent studies include cerebral blood flow reduced in some brain regions [15–18], ventricular cerebrospinal fluid lactate increased [19], concentrations of corticosteroid declined combined with feedback of the hypothalamic-pituitary-adrenal (HPA) axis enhanced [20–22], brain-derived neurotrophic factor (BDNF) decreased [23, 24], serotonergic neurotransmitter system altered [25–27], and brain cytokines system activated [28, 29].