1. Department of Epidemiology, and
2. Department of Environmental Health Sciences, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA
3. Department of Epidemiology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA
4. Department of Pathology, Division of Microbiology, Johns Hopkins Hospital, Maryland, USA
5. Department of Environmental and Occupational Health, George Washington University, Washington, DC, USA