1. Theoretical Biology and Biophysics Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
2. Department of Microbiology, University of Tennessee, Knoxville, Tennessee 37996
3. Weatherall Institute of Molecular Medicine, Oxford University, Oxford, United Kingdom
4. Duke University Medical Research, Duke University, Durham, North Carolina 27710
5. Department of Medicine and Microbiology, University of Alabama at Birmingham, Birmingham, Alabama
6. The Jenner Institute, University of Oxford, Compton, United Kingdom
7. Santa Fe Institute, Santa Fe, New Mexico 87501