REFERENCE:Sanjuán R, Illingworth CJR, Geoghegan JL, Iranzo J, Zwart MP, Ciota AT, Moratorio G, Gago-Zachert S, Duffy S, Vijaykrishna D. 2021. Five Challenges in the Field of Viral Diversity and Evolution. Frontiers in Virology. 1. doi:10.3389/fviro.2021.684949.