1. Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK
2. Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK
3. Centre for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, 3720 BA, The Netherlands
4. Gastrointestinal Disease and Zoonoses Department, Health Protection Scotland, Glasgow G3 7LN, UK
5. Centre for Infections, Health Protection Agency, London NW9 5HT, UK