1. University of Southampton, School of Medicine, Mailpoint 805, Southampton General Hospital, SO16 6YD, U.K., Imperial College, St. Mary's Medical School, Norfolk Place, London, W2 1PG, U.K., University of Leeds, Leeds, LS2 9JT, U.K., and London School of Hygiene and Tropical Medicine, Keppel Street, London, WC1E 7HT, U.K.