The Mystery of the SARS Virus: How Is It Spread?

Stefan Lovgren
for National Geographic News
April 9, 2003
When Block E of the Amoy Gardens apartment complex in Hong Kong suddenly turned into the latest epicenter of severe acute respiratory syndrome (SARS) last week, health officials were puzzled. How could a virus that is usually transmitted by a simple cough or a sneeze infect hundreds of residents by raging through an apartment building with no central ventilation?

Until then, it seemed that SARS spread mainly through close contact with healthcare workers or family members. The outbreak in the Hong Kong apartment building shows that SARS, and the way it spreads, is probably more complex than previously thought.

The epidemic, which has infected at least 2,722 people as of today and killed over a hundred, mainly in Asia, has triggered an unprecedented international campaign to identify the cause of the disease and stop its spread.

So far, 149 suspected cases have been reported in the United States, but no one has died. The Centers for Disease Control and Prevention in Atlanta has called SARS "an urgent global public health threat."

But the death toll doesn't explain why SARS has caused such a huge health scare, stalling airlines and even prompting economists to trim growth forecasts for parts of Asia. After all, a common disease like the influenza kills 20,000 people every year; 200 times as many people who have died of SARS.

Instead, say experts, it's the mystery surrounding the disease that's causing such fear. "This virus is brand new," said Dr. Lee Harrison, associate professor of epidemiology at University of Pittsburg. "There's very little we know about it."

Symptoms include high fever, aches, dry cough, and a shortness of breath. Chest x-rays have shown what doctors call ''atypical pneumonia'' in a lower lobe of a lung. The severity of the disease varies, but nearly four percent of people infected with SARS have died.


The epidemic probably started in November last year in southern China's Guangdong province when a businessman became ill with an unusual case of pneumonia. Doctors could not identify what was making him sick. The four health workers who treated him also fell ill. Now, it appears this businessman was the first person to contract SARS.

Understanding how the epidemic is spreading is the key to controlling it. At first, health officials seemed confident that SARS spreads only through so-called droplet transmission—when people infected with SARS cough or sneeze, they release droplets into the air which are then breathed in by someone else. Because of their large size, droplets reach only about three feet (0.9 meter) before falling to the ground.

But the rapid spread of SARS through the Amoy Gardens apartment complex, for example, suggests a more troublesome scenario: that the virus can also spread through airborne transmissions, that is small respiratory aerosol particles that can be breathed in by people. Such particles can hover in the air and travel over a far greater distance than the droplets.

The spread of the epidemic has been erratic. Experts warn that some people may be so-called "superspreaders," or more prone than others to transmit the virus. Superspreaders would carry more of the infectious virus in their respiratory secretions. They probably cough a lot. In an environment that is closed-in, like a hotel or an airplane, they can easily spread the disease.

One superspreader may have been a 64-year-old doctor in China who traveled to Hong Kong in late February. Before he died in March, he is believed to have infected eight people on his floor at Hong Kong's Metropole Hotel.

"It's very likely this virus is usually spread by droplet transmission, but from time to time it may be embedded in a "superspreader" who may also spread the virus through airborne transmissions," said Dr. William Schaffner, chairman of the department of preventative medicine at Vanderbilt University, in Nashville, Tennessee.

One theory suggested by medical investigators is that infected people may carry the disease without suffering extreme symptoms. Healthy carriers of a disease are not unusual, the most famous of whom was Mary Mallon. Popularly known as Typhoid Mary, she spread typhoid fever in the United States without suffering symptoms herself.

A Brand New Virus

Tests have linked SARS to a family of viruses known as the coronaviruses (though scientists are still awaiting confirmation). Until now, coronaviruses were believed to produce only minor illnesses in people, like colds, diarrhea and other intestinal disorders. In cats, dogs, chickens, pigs, and cattle, coronaviruses can cause severe and often fatal illnesses.

But SARS appears to be a brand new virus, separate from the known coronaviruses. How it has become so deadly is still a mystery. One possibility is that the virus jumped from an animal. Such leaps have happened before. The hendra virus spread from horses to people in Australia, while the nipah virus went from pigs to humans in Malaysia. However, neither virus then spread from person to person.

Another possibility is that the human coronavirus acquired genes from another, more virulent virus. Coronaviruses are able to capture stray bits of genetic material from related viruses and weave them into their own genomes, a feat biologists call recombination. Such a recombination creates new viruses, and in theory could turn a benign microbe into a biological time bomb.

Experts say SARS could grow less virulent as it reproduces inside the human body, or it may grow worse. "Until there is DNA sequencing of this virus, it's impossible to determine its origin," said Harrison.

A Wake-up Call

The long-term course of SARS is impossible to predict, experts say. While some dangerous new infections have burned themselves out, others, like HIV, have become global disasters. There are signs that disease rates are dropping in China. But they may be on the rise in Hong Kong and elsewhere.

"This [is] a wake-up call to the United States and to other countries around the world regarding the challenges that emerging infectious diseases can pose," warned James Hughes, director of the National Center for Infectious Diseases and the Centers for Disease Control and Prevention in Atlanta, Georgia.

Some experts suggest that if this outbreak dies down, the virus could pop up again with no warning or it might follow a seasonal pattern, like the flu.

"The world of infections are in constant flux and turbulence," said Schaffner. "The only constant is change. We should expect from time to time for new viruses to emerge and new epidemics to occur."

© 1996-2008 National Geographic Society. All rights reserved.