SARS is caused by a virus referred to as "SARS-CoV" from the coronavirus genus; SARS-CoV means severe acute respiratory syndrome-associated coronavirus. Many coronaviruses infect animals and humans, and the common cold is caused by some coronaviruses and several other viruses. However, the SARS-CoV virus had never been identified before 2002. This was not entirely surprising because there are many types of coronaviruses, and they are known to mutate easily.

Although scientists are not certain, it has been suggested that the SARS virus originated in wild bats and then spread to palm civets or similar mammals. The virus then mutated and adapted itself in these animals until it eventually infected humans. There was ample opportunity for the virus to come into contact with humans. Bats serve as a food source in parts of Asia, and their feces are sometimes used in folk medicines. Civets are cat-like mammals that live in the tropics of Africa and Asia and produce musk from their scent glands, which is used in perfumes. Civets are also hunted for meat in some parts of the world. These animals could easily transmit the virus to humans.

SARS-CoV is spread from person to person through respiratory secretions. SARS often affected people caring for a sick individual and spread readily through health-care facilities until infection-control measures were established. SARS-CoV was isolated from many hospital surface areas, including elevator buttons, likely contributing to the spread of the disease among healthcare workers. During the outbreak, one in about every 20 infected people was a health-care worker who cared for a patient with SARS; nearly 2,000 health-care workers became ill.