Health officials on three continents are grappling with a rapidly spreading epidemic of a severe respiratory ailment known as SARS. Here's a quick look at who's at risk and other basic questions about the disease.

Q. What is SARS?

A. SARS stands for severe acute respiratory syndrome. It's a new disease that doctors still don't know much about.

Q. What are the symptoms of SARS?

A. They are a lot like pneumonia or the flu. People get a very high fever -- at least 104 degrees. They also usually have shortness of breath or other problems breathing and a dry cough. Some people get other symptoms, including a headache, stiff or achy muscles, a loss of appetite, fatigue, a rash and diarrhea.

Q. How do you get SARS?

A. It seems that you have to have very close contact with someone who has it. Almost all the people who have gotten SARS have either been hospital workers who cared for sick people or members of a victim's family. Doctors believe that it is spread by tiny droplets that get airborne when someone sneezes or coughs, or by contact with other bodily fluids such as blood. The people who have gotten SARS outside of Asia have all either recently traveled to Asian countries where it is spreading or had close contact with someone who recently returned from there.

Experts don't think it's easy to catch SARS from sitting next to a sick person on a plane, but they are investigating one incident in which Chinese tourists may have gotten infected by flying on a plane with an infected man.

Q. Where is it spreading the most?

A. The disease has hit hardest in China, especially in Hong Kong and the southern province of Guangdong. But there has also been a number of cases in Hanoi and in Singapore. The outbreak nearest to the United States has been in Toronto.

Q. Have any Americans gotten sick?

A. The federal Centers for Disease Control and Prevention says it is investigating at least 59 suspected cases in 22 states.

Q. Are people in this area at risk?

A. No one can yet predict how the epidemic will unfold in the future, but at this point there seems to be little risk unless you are in contact with people who have traveled to affected areas or have been there yourself. Four possible cases have, however, been reported in Virginia.

Q. How can I protect myself?

A. The best way is to avoid traveling to places where the disease is most common and avoid close contact with someone who appears to have the disease. Hospital workers who have started wearing masks and gloves have not gotten sick.

Q. Can SARS be treated?

A. Antibiotics don't seem to work, which is usually the case with virus-caused diseases. One antiviral drug known as ribavirin may help, but doctors aren't sure yet.

Q. How dangerous is the disease?

A. Between 80 percent and 90 percent of patients get better on their own in about a week. The other 10 percent to 20 percent get worse, with many ending up in intensive care and requiring mechanical ventilators to help them breathe. About half of those people die.

Q. Who is most at risk?

A. People over age 40 and those who have other medical problems, such as heart or liver disease, seem to do the worst.

Q. What causes SARS?

A. Scientists are not sure, but they have found two previously unknown viruses in patients. One is from the coronavirus family of viruses, the other from a different family called paramyxoviruses. Paramyxoviruses cause many different diseases, including mumps, measles and respiratory illnesses. Coronaviruses usually just cause the common cold.

Q. What is the incubation period?

A. Between two and seven days after exposure, with most people getting symptoms in three to five days.

Q. Where did SARS come from?

A. The disease is believed to have first emerged in Guangdong province in China in November and then spread to Hong Kong and elsewhere.

Q. Should I avoid traveling?

A. The World Health Organization is not recommending that people change travel plans but urges them to be on the lookout for symptoms. The CDC yesterday said that anyone who has "nonessential" plans to go to mainland China, Hong Kong, Singapore and Hanoi "may wish to postpone their trips until further notice."

Q. Could this be bioterrorism?

A. Health officials aren't ruling anything out, but they think this is something that occurred naturally, perhaps when a virus that usually only makes animals sick changed somehow and became able to make people sick.

Severe acute respiratory syndrome (SARS) has hit hardest in China, where cases were first reported.