1. Institute for Advanced Study, Olden Lane, Princeton, NJ 08540, USA
2. Centre for Mathematical Biology, Mathematical Institute, University of Oxford, 24-29 St Giles', Oxford 0X1 3LB, UK
3. The Wellcome Trust Centre for the Epidemiology of Infectious Disease, Department of Zoology, University of Oxford, South Parks Road, Oxford 0X1 3PS, UK
4. Institute of Medical Microbiology and Immunology, The Panum Institute, 3C Blegdamsvej, DK–2200 N Copenhagen, Denmark
5. Laboratory of Retroviral Pathogenesis, AIDS Vaccine Program, SAIC Frederick, National Cancer Institute–Frederick Cancer Research and Development Center, Building 535, Fifth Floor, Frederick, MD 21702, USA