1. Division of Infectious Diseases, Departments of Pediatrics1 and
2. Medicine,2 University of California—San Francisco, San Francisco, California 94143-0654;
3. Department of Microbiology and Immunology, Stanford University, Stanford, California 943053;
4. Infectious Disease Unit, Massachusetts General Hospital, Boston, Massachusetts 021144; and
5. Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 021155