Researchers say they now know why influenza B is limited to humans, a discovery that could lead to new drugs to fight seasonal flu epidemics. The findings also help explain why influenza B cannot be as virulent as A strains that incorporate new genes from influenza viruses that infect other species. The devastating flu pandemic of 1918, the pandemics of 1968 and 1977, and the avian influenza that emerged in the middle of the last decade were caused by influenza A viruses.
