1. Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA
2. Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA
3. Epidemiology Branch, Division of Viral Diseases, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30033, USA
4. Woodrow Wilson School of Public and International Affairs, Princeton University, Princeton, NJ 08544, USA