Diagram representing criteria for selection of case and control subjects to evaluate risks for SARS-CoV infection stemming from hospital exposure to the Hanoi index patient. (*) SARS cases were confirmed by serologic testing, viral culture, or RT-PCR performed on specimens obtained from persons with clinically compatible illness. (†) Excluded as study cases were SARS cases among the staff who were unlikely to have contracted infection from the index case (i.e., illness onset after March 5th, 2003 or seroconversion > 18 days after last exposure to the index patient); included as study cases are those SARS cases among the staff who had illness onset on or before Mar 5th, or seroconversion within 18 days of last exposure to the index case. (‡) Included as study controls were non-cases demonstrated to be negative for SARS-CoV antibody at least 18 days after last exposure to the index case. Potential control subjects were excluded from the analysis if no serologic specimen was collected from them or if the specimen was collected too early to assess final outcome status from exposure to the index case (i.e., within 18 days of last exposure).