1. From the Partners AIDS Research Center and Infectious Disease Unit, Massachusetts General Hospital and Harvard Medical School, Boston, MA; the Nuffield Department of Clinical Medicine, Peter Medawar Building for Pathogen Research, Oxford University, Oxford, United Kingdom; the Gastrointestinal Unit, Massachusetts General Hospital and Harvard Medical School, Boston, MA; the Lemuel Shattuck Hospital, Jamaica Plain, MA; and the Howard Hughes Medical Institute, Chevy Chase, MD.