The bug got out of the box at the National Institute of Virology in Beijing.
Two of the recently reported cases were conducting research at the laboratory: a 26 year old female postgraduate student from Anhui Province, and a 31 year old man.
The dates of symptom onset in the 2 cases were separated by 23 days, suggesting that more than one opportunity for exposure may have occurred in the laboratory from mid-March through early April 2004.
Authorities have closed the virology institute and placed its more than 200 employees under medical observation. Numerous environmental samples from the laboratory have been taken to help assess possible sources of contamination, and these samples will be shared with WHO.
From the information currently available, there is now evidence of 3 generations of transmission.
The 1st generation involved the index case from Anhui, and possibly a 2nd "1st-generation" case, the postdoctoral student from Beijing whose onset of illness was 23 days after the onset of illness of the index case from Anhui.
The 2nd generation included the mother of the index case from Anhui and the nurse who cared for the index case from Anhui while she was hospitalized in Beijing, and possibly the physicians who cared for her while hospitalized in Anhui.
The 3rd generation of cases are the family members of the nurse (from the 2nd generation), a patient hospitalized in the same ward as the nurse before SARS was identified as a possible diagnosis for the nurse's illness, and a close contact of one of the physicians who treated the index patient in Anhui.
This pattern of nosocomial spread due to lack of early identification of SARS as a possible diagnosis is very reminiscent of the outbreak in 2003.
As is the failure to restrict travel soon enough.