1. Department of Genitourinary Medicine, Clinic 1A, Box 38, Addenbrookes Hospital, Cambridge University Hospitals NHS Trust, Cambridge CB2 0QQ
2. Department of Microbiology, Health Protection Agency, Box 236, Addenbrookes Hospital, Cambridge University Hospitals NHS Trust, Cambridge
3. Statistics Unit, Statistics, Modelling and Economics Department, Health Protection Services, Colindale, London, UK