Thanks to the students and collaborators who have helped me to get more and more interested in complex networks over the years, and whose knowledge and enthusiasm have been so motivating: Davide Cellai, Xue Guo, Peter Mann, John Mitchell, Stefan Nixon, Martynas Noreika, Aleksejs Sazonovs, Saray Shai, Anne Smith, Mike Pitcher. The fact that I still understand so poorly remains my fault.

Thanks to the friends and colleagues who asked about epidemic modelling and were then sucked into commenting, proof-reading, and listening to half-formed explanations: Diego Arenas Contreras, Muffy Calder, Simone Conte, Lisa Dow, Lei Fang, Ian Gent, Fenella Hayes, Julie McCann, Conor Muldoon, Riccardo Romano, Juan Ye. No good deed goes unpunished.

Thanks to the University of St Andrews for providing both a stimulating academic home and access to the computing resources needed to crunch the numbers.

This book would not exist without covid-19. Whether that is something to thank it for, or something further to blame it for, I leave up to you.