On 6 March 2003, the Singapore Ministry of Health was notified of a cluster of atypical pneumonia in three patients with a history of travel to Hong Kong (Hsu et al., 2003). These three female travelers had stayed at the Metropole Hotel on the same floor as a Chinese physician later diagnosed with severe acute respiratory syndrome (SARS) (Hsu et al., 2003; Peiris et al., 2003). After returning to Singapore, one of the travelers (index A) who developed fever on February 25 was hospitalized at Tan Tock Seng Hospital on March 1, and was managed initially for straightforward community-acquired pneumonia. The other two travelers were also admitted with similar symptoms. Shortly thereafter, clusters of cases emerged in three separate wards, all traceable to the first imported case. By the time index A was isolated on March 6, she had already infected 22 persons, comprising ten health care workers, two inpatients, seven visitors, and three family members. One of the infected health care workers (index case B), with onset of symptoms on March 7 and a provisional diagnosis of dengue fever, was later admitted on March 10 to Ward 8A. At the ward she in turn infected 21 persons, including an inpatient with ischemic heart disease and diabetes mellitus, before she was isolated on March 13 (Wilder-Smith et al., 2004b). The inpatient (index case C) had been admitted on March 10 with fever, community-acquired pneumonia, and gramnegative bacteremia. When she developed heart failure on March 12, she was transferred to Ward 6A (the coronary care unit) and mechanically ventilated. However, she was isolated only on March 20 when SARS was suspected. By that time, 21 health care workers and 5 family members had become infected (Wilder- Smith et al., 2004b). A total of 109 cases were epidemiologically linked to index A. Intra-hospital transmission at Tan Tock Seng Hospital was interrupted by April 12, the date of onset of the hospital’s last case. Despite the institution of very rigorous infection control measures at Tan Tock Seng Hospital, SARS spread to four other health care institutions (Singapore General Hospital, National University Hospital, Changi General Hospital, and Orange Nursing Home – the last two are grouped together in Fig. 1 and a vegetable wholesale market (Gopalakrishna et al., 2004) (Fig. 1).