SGCG and infection: The immunosuppression induced by PCV2 mainly manifests as a decrease in phagocytosis and the antigen-presentation function of alveolar macrophages and dendritic cells; the disturbance of the cytokine-expression profile, such as the significant increase in IL-10 expression; and the decrease in type I or II interferon and IL-12 expression, which create favorable conditions for the secondary infection of other pathogenic microorganisms [37,38,39,40,41].