SARS is caused by a type of coronavirus, which usually produces colds in humans but severe conditions such as pneumonia and diarrhoea in animals. The SARS virus was thought to have mutated to allow it to jump the species barrier from animals to humans. Analysis of the virus genome suggests that it is a recombinant of a mouse hepatitis virus and a bird bronchitis virus. The incubation period of SARS varies but is on average between two and ten days. The risk of death increases with age and the mortality rate is as high as 50% in those over 60 years of age.

The first outbreak of SARS is thought to have been near Foshan in the southern Chinese province of Guangdong in November 2002, but the disease only came to public attention in February 2003 as it rapidly spread worldwide due to the use of air travel. The disease was identified and named by Carlo Urbani, an infectious disease expert working for the WHO in Bangkok, Thailand, who was called to Hanoi, Vietnam, in February 2003 to examine a US industrialist suffering from an unidentified illness. By April, scientists, most notably from Canada, were able to link SARS to the coronavirus family.

The disease spreads by victims being in close proximity to an infected person, who can spread the disease by coughing, or being in an area where they have left residues such as exhaled droplets or other bodily fluids. The virus can be contracted by touching objects that have contaminated droplets on them, such as door handles, or being in an enclosed space where droplets could persist, such as hotel lifts.

By the end of the 2003 outbreak, over 8,000 victims of SARS had been reported globally, of whom over 700 died since the outbreak began. China, Hong Kong, and Taiwan were the worst-affected countries. Although other cases were reported globally, the only major hot spots were Toronto, Canada, which had over 140 reported cases and over 25 deaths, and Singapore, which had over 300 cases and over 30 deaths.

In 2003 scientists announced the discovery of a gene that causes susceptibility to infection by the SARS virus. In their study of human leukocyte antigen (HLA) proteins, the scientists discovered that variation in a gene that produces the protein HLA-B*4601 was widespread among people indigenous to South China, the region where SARS originated. People who possessed the gene variation showed a significantly more severe reaction to the SARS virus than those who did not. The incidence of SARS infections is very low among those populations that do not possess the HLA-B*4601 gene variation, such as the majority of Taiwanese and Europeans.