 The influenza virus has also caused several pandemic threats over the past century , including the pseudo @-@ pandemic of 1947 ( thought of as mild because although globally distributed , it caused relatively few deaths ) , the 1976 swine flu outbreak and the 1977 Russian flu , all caused by the H1N1 subtype . The world has been at an increased level of alert since the SARS epidemic in Southeast Asia ( caused by the SARS coronavirus ) . The level of preparedness was further increased and sustained with the advent of the H5N1 bird flu outbreaks because of H5N1 's high fatality rate , although the strains currently prevalent have limited human @-@ to @-@ human transmission ( anthroponotic ) capability , or epidemicity . 
