1. Infection Control Unit, Massachusetts General Hospital, Boston, MA, USA (ESS, DCH)
2. Division of Infectious Diseases, Department of Medicine, Massachusetts General Hospital, Boston, MA, USA (ESS, RPW, DCH)
3. Medical Practice Evaluation Center, Massachusetts General Hospital, Boston, MA, USA (ESS, ER, TH, RPW)
4. Harvard Medical School, Boston, MA, USA (ESS, RPW, DCH)
5. Massachusetts General Hospital Biostatistics Center, Boston, MA, USA (HL)