1 Medical Research Council (MRC) Centre for Global Infectious Disease Analysis, School of Public Health, Imperial College London, London, UK.
2 School of Biology and Ecology, University of Maine, Orono, ME 04469, USA.
3 Medical Research Council (MRC) Centre for Global Infectious Disease Analysis, School of Public Health, Imperial College London, London, UK. Electronic address: matthew.fisher@imperial.ac.uk.