[1] Univ Cambridge, Dept Pathol, Cambridge CB2 1QP, England
[2] Univ Cambridge, Sch Vet, Ctr Vet Sci, Cambridge, England
[3] UCL Royal Free & Univ Coll Med Sch, Dept Virol, London, England
[4] Univ Alabama, Birmingham, AL USA
[5] Univ British Columbia, Vancouver, BC V5Z 1M9, Canada
[6] Childrens Hosp, Med Ctr, Div Infect Dis, Cincinnati, OH 45229 USA