CALCA and viral infectious disease: For multiple years, the relationship between PCT and infection has been recognised by the academic community, mainly when severe bacterial infections occur in the body, a marked serum PCT upregulation can be observed; nevertheless, in inflammatory responses and mild infections due to viral infections, autoimmune diseases, organ transplant rejection, etc., serum PCT upregulation is not remarkable; thus, PCT possess high specificity and sensitivity in diagnosis and differentiation of bacterial infections.