US investigating potential human-to-human bird flu transmission Medical XpressFour more US health workers show symptoms after contact with Missouri bird flu patient ReutersMissouri investigates more possible human-to-human H5N1 avian flu spread University of Minnesota Twin CitiesScientists race to investigate possible human transmission of H5N1 in US outbreak The TelegraphPossible Cluster of Human Bird-Flu Infections Expands in Missouri The New York Times