1. Department of Biological Sciences, Purdue University, 915 W. State Street, West Lafayette, Indiana 47907-2054
2. Baker Institute for Animal Health and Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, New York 14853
3. Department of Molecular Biology, Cell Biology and Biochemistry, Brown University, Providence, Rhode Island 02912
4. Departments of Pediatrics and of Microbiology and Immunology, Weill Medical College of Cornell University, 515 East 71st St., Box 309, New York, New York 10021