CRP and infection: Important for the pathogenesis of PCS is the severity of the initial infection, since patients with severe COVID experience a systemic proinflammatory immune response with elevated levels of IL-1, IL-2, IL-6, and IL-7, tumor necrosis factor (TNF)-a, GM-CSF, macrophage inflammatory protein 1-a, C-reactive protein (CRP), ferritin, and D-dimer.