1. Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, UK.
2. Centre for Mathematical Biology, University of Oxford, Oxford OX1 3LB, UK.
3. Weatherall Institute of Molecular Medicine, University of Oxford, Oxford OX3 9DS, UK.
4. Department of Mathematics and Institute of Applied Mathematics, University of British Columbia, Vancouver, British Columbia V6T 1Z2, Canada.