1. Infectious Disease Division, Massachusetts General Hospital, Boston, MA 02114, USA.,
2. Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, MI 48109, USA.,
3. Unit for Laboratory Animal Medicine, University of Michigan Medical School, Ann Arbor, MI 48109, USA.,
4. Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115, USA.