Atypical pneumonia cases, later characterized as SARS, first occurred in the Guangdong Province of China in mid-November 2002. Early data suggested a possible zoonosis, with the earliest SARS cases detected among workers dealing with exotic food animals. In subsequent studies, SARS-like coronaviruses were detected or isolated from two wild animal species in live animal markets, the masked palm civets (Paguma larvata) and raccoon dogs (Nyctereutes procyonoides), although the natural animal reservoir remains uncertain. Further studies have documented that nonhuman primates, ferrets, domestic cats, mice, and hamsters are also susceptible to experimental SARS coronavirus (Fouchier et al., 2003; Guan et al., 2003; Martina et al., 2003; Glass et al., 2004; McAuliffe et al., 2004; Roberts et al., 2005); pigs in China were naturally infected with a human-like SARS strain lacking the 29-nt insertion (Chen et al., 2005). Conversely, civets are susceptible to infection and disease with at least two human strains of SARS CoVs: one early human isolate (GZ01) with the 29-nt insertion like the civet strains, and one later human strain (BJ01) without the 29-nt insertion (Wu et al., 2005). Similar exotic animal markets also provided a breeding ground for recent influenza outbreaks in Hong Kong. The unsanitary conditions in live animal markets in China (and elsewhere) foster an environment conducive to the emergence of new zoonotic and animal diseases and likely played a role in SARS transmission from animals to humans (Peiris et al., 2004; Xu et al., 2004).
Global spread of the SARS epidemic was triggered on February 21, 2003, by a superspreading event in the Metropole Hotel in Hong Kong by an infected physician from Zhongshan University in China. Within 24 hours, he infected others at the hotel, who then carried SARS to Singapore, Vietnam, Canada, Ireland, and the United States, besides elsewhere in Hong Kong. Based on WHO estimates, this superspreader initiated a chain of infection involving nearly half of the 8,000 cases in more than 30 countries. On March 12, 2003, the World Health Organization (WHO) issued a global alert describing atypical pneumonia cases (severe acute respiratory syndrome or SARS) in Hong Kong and Vietnam and initiated worldwide surveillance. In an unprecedented move on March 15, WHO issued a travel advisory regarding high-risk areas where SARS outbreaks had been detected. The agency continued to issue travel advisories and advise airline passenger screening from high-risk areas through mid-April 2003.