1. Environmental and Enteric Diseases Department, Health Protection Agency Centre for Infections, 61, Colindale Avenue, London, NW9 5EQ, UK
2. Also at: School of Medicine, Health Policy and Practice, University of East Anglia, Norwich, NR4 7TJ, UK
3. Consultant in Communicable Disease Control, North East and Central London Health Protection Unit, 68 Leadenhall Street, London, EC3A 3DH, UK
4. Statistics Department, Health Protection Agency Centre for Infections, 61, Colindale Avenue, London, NW9 5EQ, UK