The difference in thrombotic and hemorrhagic phenotypes has been attributed to variability in levels of Factor VIII and von Willebrand factor, where ABO status may explain as much as 30% of this variability (Franchini et al., 2012; Orstavik et al., 1985; Germain et al., 2015; Lindström et al., 2019; O'Donnell et al., 2002) Other prominent examples include associations with risks of a number of infectious diseases, to the extent that the allele distribution of the blood group antigens has evolved to reflect some areas endemic to these infectious diseases (Franchini and Bonfanti, 2015). The gene discussed is VWF; the disease is infectious disease.