1 Guy's King's and St Thomas' School of Medicine, King's College London, London SE1 1UL, UK; London School of Hygiene & Tropical Medicine, London, UK. Electronic address: faheem.ahmed@kcl.ac.uk.
2 St George's Hospital, London, UK.
3 King's College Hospital, London, UK.
4 London School of Hygiene & Tropical Medicine, London, UK.