This is attributed to original antigenic sin, the idea that when encountering a pathogen, the immune system uses immunological memory based on a previous response to a similar antigen. Recent studies have shown that there is cross-immunity against viruses in the same hemagluttinin (HA) group. HA group 1 contains human seasonal subtypes H1, H2, and avian H5; group 2 contains seasonal H3 and avian H7. In 1968, H3N2 caused the pandemic known as the Hong Kong flu. Prior to 1968, most people were first exposed to viruses with H1 or H2 proteins, which would also protect them from viruses with H5 proteins, such as H5N1. Most people born after the Hong Kong flu were likely first exposed to viruses with H3 proteins, which would also protect them from viruses with the H7 protein, such as H7N9.