CD4 and viral infectious disease: As far as we know, the lower CD4+/CD8+ ratio could be attributed to the persistent inflammation and immunosenescence caused by viral infection (Sainz et al., 2013) and can also be used as a biomarker for T-cell activation to characterize the migration of T cells into the CNS after HIV infection and the production of inflammatory cytokines, which can indirectly lead to white matter damage.