1. Centre for Immunity, Infection and Evolution, University of Edinburgh, Edinburgh EH9 3FL, UK.
2. Fogarty International Center, National Institutes of Health, Bethesda, MD 20892, USA.
3. Wellcome Trust Sanger Institute, Cambridge CB10 1RQ, UK.
4. Division of Infection & Immunity, University College London, London WC1E 6BT, UK.