1. From the Vascular Physiology Unit (M.C., A.E.D., J.P.H., P.F., J.E.D.) and Infectious Diseases and Microbiology Unit (N.J.K.), Institute of Child Health, UCL University, London, United Kingdom; and the Unit of Paediatric and Perinatal Epidemiology (M.T., S.L., A.N., G.D.S., J.G.), Department of Community Based Medicine, University of Bristol, Bristol, United Kingdom.