The debate over studying avian influenza – or H5N1 virus – transmissibility returns pandemic risk to the fore of public anxiety. Regardless of the specific agent, influenza or otherwise, a robust plan for preparedness and response must contend with multiple uncertainties. Containing transmission is key. The 1918 Spanish flu wiped out 30 per cent of the world’s population at the advent of modern transportation that, by the start of the 20th century, offered global coverage. Infected passengers of ships and trains spread the flu; moreover, severe epidemics occurred in shipyards and among railway personnel.