1. Virology Department, Hôpital Bichat, 75877 Paris 18, France.
2. Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721.
3. Infectious diseases Department, Hôpital Bichat, 75877 Paris 18, France.
4. School of Biological Sciences, University of Manchester, Manchester M13 9PT, UK.
5. Institut Pasteur de Dakar, Dakar, Sénégal.