1 From the Renal-Electrolyte and Hypertension Division (P.P.R.), the Departments of Biostatistics and Epidemiology (P.P.R., D.S.G.) and Surgery (P.L.A.), and the Divisions of Infectious Diseases (E.A.B.) and Gastroenterology (D.S.G.), Perelman School of Medicine, University of Pennsylvania, Philadelphia.