Dept. of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, United Kingdom.|Dept. of Mathematical Sciences, Durham University, Durham, United Kingdom.|School of Medicine, Pharmacy and Health, Durham University, Durham, United Kingdom.|Dept. of Veterinary Medicine, University of Cambridge, Cambridge, United Kingdom.|School of Mathematics and Statistics, University of Sheffield, Sheffield, United Kingdom.|Medical Research Council/Uganda Virus Research Institute, Uganda Research Unit on AIDS, Entebbe, Uganda.|Dept. of Mathematical Sciences, Durham University, Durham, United Kingdom.|Dept. of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, United Kingdom.