1. Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
2. National Influenza Center and Department of Virology, Erasmus Medical Center, Dr. Molewaterplein 50, 3015GE Rotterdam, Netherlands.
3. Theoretical Division, T-13, MS B213, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.