GI.1–RHDV, including GI.1a–RHDVa, which infects adult and young rabbits but over 4–5 weeks of age and causes rabbit haemorrhagic disease, has also been shown to have a high affinity for HBGA antigens present in the gastrointestinal and respiratory tracts of these animals, while GI.2–RHDV2/b shows an analogous affinity to these antigens, not only in adult rabbits but also in young rabbits up to 4 weeks of age and hares, which explains the higher morbidity and incidence of GI.2–RHDV2/b virus infection in these animals [14,109,137,139,160,161,162,163]. This evidence concerns the gene HBG1 and viral infectious disease.