1. Wellcome Trust Centre for the Epidemiology of Infectious Disease, Department of Zoology, Oxford University, Oxford OX1 3FY,1 and
2. Wellcome Trust Centre for Clinical Tropical Medicine, Nuffield Department of Medicine,2 and
3. Department of Pathology and Bacteriology,3 Oxford University, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom