Within the serotonergic alterations presented by patients with ME/CFS, it has been suggested that viral infection may cause damage to the blood–brain barrier, leading to the entry of viral genetic material that activates TLR3 receptors in microglia, leading to increased release of IL-1β and TNF-α, which increases SERT expression and activity in astrocytes and, therefore, increases 5-HT reuptake and its degradation to the metabolite 5-HIAA, thereby reducing extracellular levels of 5-HT [271, 285–288]. This evidence concerns the gene TNF and myalgic encephalomeyelitis/chronic fatigue syndrome.