SARS typically begins with flu-like signs and symptoms — fever, chills, muscle aches and occasionally diarrhea. After about a week, signs and symptoms include:

• Fever of 100.5 F (38 C) or higher
• Dry cough
• Shortness of breath

SARS is caused by a strain of coronavirus, the same family of viruses that causes the common cold. Until now, these viruses have never been particularly dangerous in humans, although they can cause severe disease in animals. For that reason, scientists originally thought that the SARS virus might have crossed from animals to humans. It now seems likely that it evolved from one or more animal viruses into a new strain.

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 face-to-face contact, but the virus may also be spread on contaminated objects — such as doorknobs, telephones and elevator buttons.

In general, people at greatest risk of SARS have had direct, close contact with someone who's infected, such as family members and health care workers.

Most people with SARS develop pneumonia. Breathing problems can become so severe that a mechanical respirator is required. SARS is fatal in some cases, often due to respiratory failure. Other possible complications include heart and liver failure.

People older than the age of 60 — especially those with underlying conditions such as diabetes or hepatitis — are at highest risk of serious complications.