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.

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