HBG1 and infection: ,13 For example, GI.1 infection is restricted to secretor-positive populations13; GII.4 infection is restricted primarily to secretor-positive populations14,15; and GII.3, GII.7, and GII.6 infection is secretor-independent, infecting both secretors and nonsecretors.15, 16, 17, 18, 19, 20 Consequently, human norovirus–HBGA interaction is strain-dependent.