Causes

SARS is caused by a strain of coronavirus, the same family of viruses that causes the common cold. Previously, these viruses had never been particularly dangerous to humans.

Coronaviruses can, however, cause severe disease in animals, and that's why scientists suspected that the SARS virus might have crossed from animals to humans. It now seems likely that that the virus evolved from one or more animal viruses into a new strain.

How SARS spreads

Most respiratory illnesses, including SARS, spread through droplets that enter the air when someone with the disease coughs, sneezes or talks. Most experts think SARS spreads mainly through close personal contact, such as caring for someone with SARS. The virus may also be spread on contaminated objects — such as doorknobs, telephones and elevator buttons.