SARS often resembles pneumonia or influenza, with signs and symptoms that include:

• Fever — a measured temperature of 100.4 F (38.0 C) or higher that may be accompanied by chills
• Headache
• General feeling of discomfort (malaise)
• Body aches
• Dry, nonproductive cough

Notably absent are signs and symptoms that usually occur with colds, such as sneezing and a runny nose.

SARS begins with a fever that usually occurs two to seven days after you've been infected, although symptoms sometimes may not appear for up to 10 days. Chills, headache, muscle soreness and a general feeling of discomfort also are common. Two to seven days later, you're likely to develop a dry cough. In some cases, SARS progresses to severe pneumonia, leading to an insufficient amount of oxygen in your blood (hypoxemia).

You're probably most contagious while you have active symptoms. It's unclear whether you can still transmit the disease to others before your symptoms begin or after they've disappeared. As a precaution, the CDC recommends that people who have recovered from SARS refrain from going out in public for 10 days after symptoms go away.

As soon as the first case of SARS was identified in mid-March 2003, scientists began searching for the cause. Early findings suggested a type of coronavirus, a group of viruses that normally cause mild respiratory problems, including the common cold.

This theory was borne out in April 2003 when two research teams discovered an entirely new strain of coronavirus in 27 people diagnosed with SARS. Scientists then grew the virus from tissue samples taken from SARS patients and were able to induce symptoms of the disease in monkeys.

Coronaviruses are a group of viruses that have distinctive crown-like spikes when viewed under an electron microscope. Until now, these viruses have never been particularly virulent in humans, although they've been linked to pneumonia in people with weakened immune systems. And they can cause severe illnesses in animals, including dogs, cats, pigs and birds. For that reason, scientists speculated that the SARS virus might have crossed from animals to humans, but it now seems likely that it evolved from one or more animal viruses into a completely new strain.

This theory got a boost in mid-April when Canadian and CDC scientists sequenced the entire genome of the suspected SARS virus. Both teams reported that the genome is unique and unrelated to any known human or animal viruses.

How SARS spreads
Most respiratory illnesses, such as colds and influenza, spread through droplets that enter the air when someone with the disease coughs or sneezes. This type of transmission can occur in two ways:

• Droplets. In droplet transmission, the infected particles are large and can travel only about 3 feet. To inhale them, you literally must be face to face with someone who's sick.

• Airborne particles. Because airborne particles are much smaller than droplets, they travel farther and linger longer in the air. As a result, you can become infected even after the person who coughed or sneezed has left the room.

Most experts think SARS spreads mainly through face-to-face contact, but unusual patterns of transmission, especially in Hong Kong, have caused them to look for additional explanations. It now seems likely that the virus can also spread through contact with contaminated objects — doorknobs, telephones and elevator buttons, for example. And because some people seem to have acquired the infection on airplanes, health officials think airborne transmission may be a possibility.

Although no proof exists to support the Chinese claim that cockroaches or rats spread the disease, it's likely that the SARS virus, which has been found in the stool of some patients, can spread in contaminated sewage.

Disease specialists are also debating whether some people with SARS, dubbed "superspreaders," might be especially contagious because of the large number of viruses expelled when they cough. Health experts suspect superspreaders of transmitting the illness to unusually large numbers of people.

In general, you're at greatest risk of SARS if you've had direct, close contact with someone who's infected, such as a roommate or family member. You're also at risk if you've recently traveled to places where SARS is widespread, including mainland China, Hong Kong, Singapore and Hanoi, have flown home from these areas, or have had close contact with someone who has traveled or flown home from high-risk parts of Asia.

Doctors and hospital workers who treated SARS patients before the disease was identified were some of the first SARS casualties. But stringent guidelines — including the use of gowns, gloves, goggles and N95 masks, which block 95 percent of non-oily, solid and liquid particles — are now in place to help protect health care professionals from infection.

If you've just returned from a trip to Asia or have had close contact with someone who has been in high-risk areas, the CDC suggests that you monitor your health for at least 10 days. See your doctor if you develop a cough and fever during that time. It's not known whether you can contract SARS from someone who hasn't yet developed symptoms, so health experts consider exposure to someone who isn't sick but who is at risk cause for concern.

No specific test for SARS is widely available yet, and symptoms resemble those of other respiratory infections, so diagnosing SARS can be difficult. For that reason, you're likely to have a chest X-ray to help detect lung inflammation, which may be evidence of pneumonia or respiratory distress syndrome, a condition in which the lungs are extensively inflamed and can't provide sufficient oxygen to other organs.

You're also likely to have tests to rule out other possible reasons for your cough and fever. These tests include:

• Pulse oximetry. This non-invasive test monitors the amount of oxygen in your blood. A small probe attached to your finger or ear is linked to a computerized unit that displays the percentage of oxygen in your red blood cells.

• Sputum Gram stain and culture. These tests check for the presence of bacteria that cause respiratory infections. In a stain, a sample of your sputum is stained with various dyes and then examined under a microscope. Bacteria can be identified by the color of stain they pick up as well as by their shape and size, and the results are available almost immediately. A culture is different. In this test, a sample of sputum is placed on a special medium (culture) that's incubated for one or two days, and then checked for the presence of bacteria. A positive finding on these tests tends to rule out SARS, which is a viral, not a bacterial, infection.

According to the CDC, three tests specifically for SARS are in development. Two of the tests check for antibodies your body makes in response to the specific virus, and the other detects the presence of its DNA. The drawback to the tests is that the process can take days, by which time some people with the disease may have died.

Between 10 percent and 20 percent of people with SARS become progressively worse and require intensive care. Approximately half these people develop such severe breathing problems that they need the help of a mechanical respirator.

SARS is fatal in some cases. The cause of most SARS deaths is severe lung damage, although the harm may not come from the pathogen itself but from the body's response to the infection. It's too soon to know if people who recover from SARS suffer permanent lung damage.

In spite of a concerted global effort, scientists have yet to find an effective treatment for SARS. Most people with SARS begin to improve on their own about one week after they develop signs and symptoms.

Initially, people with SARS were treated with antibiotics. Once it became clear that SARS was a viral infection, doctors switched to antiviral medications such as ribavirin and oseltamivir, sometimes in combination with steroids — a treatment that hasn't proved helpful in most cases.

A Chinese treatment using serum that contains antibodies from recovering SARS patients has run into safety problems and is available in China only on a limited and experimental basis.

Meanwhile, American researchers are testing thousands of drugs — including 30 antivirals currently on the market and about 1,000 experimental medications — in hopes that some may work against SARS. None of the drugs tested so far appears effective, and some experts think it may take years to develop a viable treatment, although the sequencing of the genome of the suspected virus may speed development of useful drugs.