1. Departments of Medicine and Microbiology,
2. Howard Hughes Medical Institute, University of Alabama at Birmingham, Birmingham, AL 35294, USA.
3. Division of HIV/AIDS Prevention–Surveillance and Epidemiology, National Center for HIV, STD, and TB Prevention, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA.
4. Institute of Genetics, University of Nottingham, Queens Medical Centre, Nottingham NH7 2UH, UK.