1. Department of Mathematics and Statistics, University of Helsinki, PO Box 68, 00014 Helsinki, Finland
2. The Wellcome Trust Sanger Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SA, UK
3. Molecular Infectious Diseases Group, Section of Paediatrics, Department of Medicine, Imperial College London, St Mary's Hospital Campus, Norfolk Place, London W2 1PG, UK
4. Center for Communicable Disease Dynamics, Department of Epidemiology, Harvard School of Public Health, Boston, MA 02115, USA