C3 and viral infectious disease: When the patient’s immune function is impaired, the lymphatic immune factor of the cells begins to decline, the secretion of immunoglobulin decreases, the phagocytosis of leukocytes is affected, and the activity of complement C3 and C4 in the serum also decreases, which makes the patient more vulnerable to external bacterial and viral infections (Tang et al., 2016; Chen, 2017; Tao et al., 2018; Xiong et al., 2019).