[1] Vet Adm Med Ctr, Gastroenterol Sect, San Francisco, CA 94121 USA
[2] Vet Adm Med Ctr, Dept Pathol, San Francisco, CA 94121 USA
[3] Vet Adm Med Ctr, Infect Dis Sect, San Francisco, CA 94121 USA
[4] Univ Calif San Francisco, Dept Med, San Francisco, CA 94143 USA
[5] Univ Calif San Francisco, Dept Epidemiol & Biostat, San Francisco, CA 94143 USA
[6] Columbia Univ Coll Phys & Surg, Dept Med, New York, NY 10032 USA