1. Center for Infectious Disease Dynamics, Department of Biology, The Pennsylvania State University, University Park, PA 16802, USA
2. Medical Research Council–University of Glasgow Centre for Virus Research, Institute of Infection, Inflammation and Immunity, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8TA, UK
3. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA
4. Fogarty International Center, National Institutes of Health, Bethesda, MD 20850, USA
5. Disease Dynamics Unit, Department of Veterinary Medicine, University of Cambridge, Cambridge CB3 0ES, UK