In November 2002, doctors in the Guangdong province of southeastern China began to see the first cases of what would become known as SARS, or severe acute respiratory syndrome. Over the next several months, 8,096 people in 26 countries contracted the new viral illness, leading to 774 deaths. Although the slow reporting of initial SARS cases helped the illness spread, globally-enforced medical practices eventually helped end the outbreak.

The reasons for the slow reporting of SARS are complicated. Doctors had never seen the viral illness before, and at first, those in Guangdong province thought the SARS cases they were seeing might be atypical pneumonia.

“Nobody was aware of it, including probably people in Beijing,” says Arnold S. Monto, a professor of epidemiology and global public health at the University of Michigan. Even after doctors began to realize that there was something new about the illnesses they were seeing, “it was kept locally for a while, which was one of the problems.”

There were also reports that officials may have encouraged doctors not to report new cases when SARS spread to Beijing. In April 2003, Time magazine obtained a letter from Jiang Yanyong, a physician at an army hospital in Beijing, alleging the actual number of SARS cases in the capital city was much higher than the official count. This turned out to be true, and Chinese officials released the real numbers that month (and also began to monitor Jiang).

SARS jumped from mainland China to Hong Kong in February 2003 when Liu Jianlun, a medical professor from Guangdong who unknowingly had SARS, checked into Room 911 at Hong Kong's Metropole Hotel. The 64-year-old professor soon became sick from the illness and went to the hospital, where he died within two weeks. But during his short stay at the hotel, he unwittingly infected several other guests. Those people then took SARS with them to Singapore, Toronto and Hanoi. (The hotel has since renamed Room 911 because of the stigma.)

“It’s a remarkable story, and it also identifies an issue that was clearly identified as a problem in 2003,” Monto says. “And that is that there were what we call ‘superspreaders’—people who seem to infect a lot of other people.” Scientists still don’t understand the biological reason why some infected people spread illness more than others do in the same situation, but they were a major factor in the spread of SARS.

“The other very important characteristic of SARS was the association or the particular problem of hospital-based infections, healthcare worker infections,” Monto says. “And this was a manifestation of the fact that sick patients and particularly very sick patients shed a lot of virus into the environment.” During the SARS outbreak, medical professionals like Dr. Liu were at an increased risk of passing the illness to people outside of hospitals.

Illnesses like SARS create panic because there are so many unknowns. In the midst of the outbreak, scientists weren’t certain if they would be able to eliminate SARS completely, or if it would become a seasonal illness like the flu, which kills hundreds of thousands of people per year. Luckily, doctors and scientists were able to completely eliminate SARS by isolating and quarantining people until the virus passed out of their system and they could no longer transmit it to others.

Because of international cooperation to isolate and quarantine people with SARS, the World Health Organization and affected countries were able to contain SARS by July 2003. After that year, the only SARS cases came from isolated laboratory outbreaks where scientists were studying the SARS coronavirus (SARS-CoV) that causes the illness. Once again, officials contained these infections through isolation and quarantines.

China and Hong Kong suffered the most during the SARS outbreak. In China, there were 5,327 cases and 349 deaths; and in Hong Kong, there were 1,755 cases and 299 deaths, according to the World Health Organization. One of the lessons of the SARS outbreak was that in the future, China needed to have more transparency between its provinces and its national government.

“SARS was an alert as to how bad it could be,” Monto says. “China has had the experience over the last 15 years of dealing with small outbreaks of avian influenza—which have not transmitted extensively, but have continued to occur in China—so they’ve learned how to deal with these.”

Preparedness for pandemics has also improved on a global level, says Anne W. Rimoin, a professor of epidemiology at the UCLA School of Public Health who specializes in the Ebola virus.

“Since the SARS outbreak, we’ve seen several other outbreaks,” she says. “We’ve had MERS, we’ve had the H1N1, that’s swine flu; we’ve had chikungunya, we’ve had Zika, and we’ve had several Ebola outbreaks since that time. So I think that the world has just gotten much better at coordinating response.”

In December 2019, a new coronavirus emerged in China. By February 2, 2020, death tolls from the new virus in mainland China exceeded that of the 2002-2003 SARS epidemic, according to China’s Health Commission.