1. Mortimer Market Centre, Camden Primary Care Trust, London, UK
2. Department of Genitourinary Medicine, St Mary's Hospital, London, UK
3. Department of Infectious Disease Epidemiology, Imperial College, London, UK
4. United Medical Schools of Royal Free and University College Hospital, London, UK
5. Health Protection Agency, Communicable Disease Surveillance Centre, London, UK
6. Health Protection Agency, Communicable Disease Surveillance Centre, London, UK; Department of Genitourinary Medicine, Guys and St Thomas' Hospital, London, UK