1. W. Harry Feinstone Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205, USA
2. †Present address: Food and Drug Administration, Silver Spring, MD 20993, USA.
3. ‡Present address: University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
4. §Present address: Ohio State University College of Medicine, Columbus, OH 43210, USA.