US investigating potential human-to-human bird flu transmission Medical XpressMissouri investigates more possible human-to-human H5N1 avian flu spread University of Minnesota Twin CitiesFour more US health workers show symptoms after contact with Missouri bird flu patient ReutersScientists 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