In addition, several studies on viral diseases in swine have investigated the increase and activity of CD8+ T cells by infecting swine with pathogens [12,13,14] and have revealed that, in particular, the damage induced by viral diseases in the early stage of weaning pigs (up to approximately 3–4 weeks) is critical even after gaining passive immunity from sow [15,16]. This evidence concerns the gene CD8A and viral load.