1. Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
2. Department of Epidemiology and Preventive Medicine, University of Maryland, 115 Howard Hall, 660 West Redwood Street, Baltimore, MD 21201, USA
3. Department of Biostatistics, Rollins School of Public Health, Emory University, 1518 Clifton Road, Atlanta, GA 30322, USA
4. National Center for Infectious Diseases, Centers for Disease Control and Prevention, 1600 Clifton Road MSG13, Atlanta, GA 30333, USA
5. Department of Biology and the Center for Disease Ecology, Emory University, 1510 Clifton Road, Atlanta, GA 30322, USA