Mystery killer. (Health Alert/Viruses).

In February, when a businessman died of a mysterious illness in Hanoi, Vietnam, doctors assumed it was pneumonia. (The first case was actually recorded last November.) By early April, the disease had a name all its own: SARS, or Severe Acute Respiratory Syndrome. At press time, it had spread to 2,722 people in 18 countries--including the U.S.--and left 106 people dead.

What is this mystery killer? Scientists suspect it may be a coronavirus--so-called because the virus (disease-causing microbe) is a chain of molecules surrounded by a spiky crown. Similar viruses cause the common cold and pneumonia.

In the early stages of SARS, patients experience fever, cough, muscle ache, and difficulty breathing. There's no treatment since antibiotics (antibacterial medicine) don't affect viruses. The good news: Experts expect 90 percent of infected patients to fully recover. "The death rate [from SARS] is lower than what we see with epidemic influenza [flu]," says Dr. Julie Gerberding, director of the U.S. Centers for Disease Control.

SARS most likely hitched a ride to humans via an infected animal. The virus is transmitted (passed between people) in tiny droplets of body fluid coughed or sneezed into the air, or by infected surfaces, such as doorknobs. So far, mostly people in close contact with infected patients have fallen ill. But doctors remain on high alert--because the world's newest virus is in many ways a science mystery. Stay tuned for a SW feature on SARS early next year.