BTN3A3 and influenza: However, the study found that some avian and swine influenza viruses have genetic mutations that allows them to pass the blocking effect of the BTN3A3 gene and infect people, the researchers concluding that all human influenza pandemics, including the 1918 Spanish flu and the 2009 swine flu pandemics, were a result of BTN3A3-resistant strains.