Despite all the above precautions and testing, blood components continue to pose a small
risk of transmission of pathogens, including viruses, bacteria, and parasites. These risks
have been estimated by means of various statistical tools. Table 1 lists known infectious
disease risks and provides some estimates of current risks. Currently unrecognized
pathogens may also emerge in time. Some studies have suggested that allogeneic blood
may have an immunomodulatory effect that may predispose transfused patients to infection
and other complications