The First and Second Waves of the 1918–1919 Pandemic. First outbreaks and foci of second waves of the pandemic are labeled as red and purple circles, respectively. The lines of spread of the first and second waves of the pandemic are labeled as purple dashed lines and red solid lines, respectively. Map images were derived and/or modified from Servier Medical Arts under a Creative Commons Attribution 3.0 Unported License. Adapted from Nicholson et al.