1. Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
2. Santa Fe Institute, Santa Fe, New Mexico 87501
3. Division of Viral Pathogenesis, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02115
4. Duke Human Vaccine Institute, Duke University Medical Center, MSRBII Building, Room 4085, P.O. Box 103020, Research Drive, Durham, North Carolina 27710