1. S. A. Levin is in the Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544, USA.
2. B. Grenfell is in the Zoology Department, Cambridge University, Downing Street, Cambridge CB2 3EJ, UK.
3. A. Hastings is in the Division of Environmental Studies, Institute for Theoretical Dynamics, and Center for Population Biology, University of California, Davis, CA 95616, USA.
4. A. S. Perelson is at Theoretical Biology and Biophysics, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.