1. J. R. Krebs, FRS, Natural Environment Research Council, Polaris House, North Star Avenue, Swindon, Wilts, SN2 1EU, UK.
2. R. M. Anderson, FRS, Wellcome Trust Centre for the Epidemiology of Infectious Disease, Department of Zoology, Oxford University, South Parks Road, Oxford, OX1 3PS, UK.
3. T. Clutton-Brock, FRS, and R. Woodroffe, Department of Zoology, Cambridge University, CB2 3EJ, UK.
4. C. A. Donnelly, Department of Zoology, Wellcome Trust Centre for the Epidemiology of Infectious Disease, Department of Zoology, Oxford University, South Parks Road, OX1 3PS, UK.
5. S. Frost, Centre for HIV Research, Room 204, Waddington Building, Kings' Buildings, West Mains Road, Edinburgh, EH9 3JN, UK.
6. W. I. Morrison, Institute for Animal Health, Compton, Nr Newbury, RG20 7NN, UK.
7. D. Young, St. Mary's Hospital Medical School, University of London, Norfolk Place, London, W2 1PG, UK.