SARS is a severe respiratory disease caused by SARS coronavirus (SARS CoV). SARS - the first newly emerged, serious and contagious illness of the 21st century - illustrates just how quickly infection can proliferate in a highly mobile and interconnected world. It's likely the disease began with a single infected person, and then spread around the globe through unsuspecting travelers.

SARS is particularly troubling because health experts know so little about it. Scientists do know that the cause is a new type of coronavirus - one of a family of viruses that in humans usually cause mild upper respiratory infections, including common colds. How the new coronavirus evolved or why it turned deadly isn't known. Nor is it clear why some people succumb to the disease and others recover. Although many who died were older adults with other health problems, SARS has also proved fatal in healthy, young adults. Adding to the uncertainty is that the symptoms are similar to those of other respiratory illnesses.

Because of the high amount of uncertainty about SARS, health officials quickly put into place an intensive global campaign to control the disease. That campaign, spearheaded by the World Health Organization (WHO), proved so successful that by July 2003 SARS had been effectively contained worldwide, and all SARS-related travel restrictions had been lifted.

That doesn't mean the disease has been eradicated, however. In 2004, some SARS cases surfaced in China. Doctors and health officials worry that an outbreak of the disease may occur again. The sense of concern surrounding SARS remains because as yet there's no known treatment.

Signs and symptoms

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

• Fever — a measured temperature of 38.0 C or higher that may be accompanied by chills

• Headache

• General feeling of discomfort (malaise)

• Body aches

• Dry, nonproductive cough

• Headache

• influenza

• Pneumonia

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.

Causes

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.

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.

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. 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 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.

• Cold

• Pneumonia

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