1. Department of Microbiology and Immunology, Dartmouth Medical School, Hanover, New Hampshire 03755.
2. Section of Infectious Diseases and International Health, Department of Medicine, Dartmouth-Hitchcock Medical Center, Hanover, New Hampshire 03755.
3. Center for the Evaluative Clinical Sciences, Dartmouth Medical School, Hanover, New Hampshire 03755.
4. Present address: University of Washington School of Medicine, Seattle, Washington 98195.