As the densely populated Northern Hemisphere enters winter, the influenza A (H1N1) virus returns and begins another wave of its spread, and these three maps show its ferocity. The map and the data behind it were compiled and produced by Henry Niman, a biomedical PhD in Pittsburgh, Pennsylvania, USA, using technology provided by Rhiza Labs and Google. The epidemic data mainly come from official information released by countries, news reports and data contributed by users, and are updated several times a day.