A highly contagious and deadly new illness sent people worldwide scrambling to cancel flights and schools were closed and sales of surgical masks spiked, a decade ago.
Severe Acute Respiratory Syndrome (SARS) was the first "new" disease of the 21st century to jump from an animal host to humans, then easily from one person to another.
It caught the world unawares and exposed health system weaknesses in an era in which people are ever more exposed to strange, new viruses lurking in hitherto undisturbed places.
"The experience from the first SARS outbreak is helping us now," virologist John Oxford from the Queen Mary University of London told AFP.
The world is currently dealing with another coronavirus like the one that caused SARS which has killed eight people since it was first detected in mid-2012.
The most important lessons cited by experts include information sharing, rapidly raising the global alert, and finding and containing the outbreak source.
"We have learnt that it is important to say what we know," Isabelle Nuttall, director of the alert and response department at the World Health Organisation (WHO) told AFP.
A lack of communication was a key criticism of the handling of the 2003 outbreak, with China accused of trying to cover it up.
Since then, the world has adopted International Health Regulations, said Nuttall, which bind all countries to report any severe, new disease with spread potential.
SARS also showed it is easy to overreact -- millions of people were screened at airports with little or no success.
Flights were cancelled as panic spread, tens of thousands of people were placed in quarantine in Asia and Canada, and hundreds of schools were closed -- all measures whose usefulness are now questioned.
The World Bank says these steps cost the world some $54 billion (41 billion euros at today's rates), much of it in lost tourism revenue.
In fact, simple hygiene -- washing your hands and masking your cough -- turned out to be a much more effective deterrent, according to Oxford.
"Everyone thought it was perhaps bigger than it was," he said of the way SARS was handled.
"Today, I doubt whether there would be this restriction on travel. Another thing the SARS outbreak told us, is that to clamp down on people's movement, it doesn't work and it causes a lot of trouble."
SARS infected about 8,000 people around the world, claiming most of its victims in Hong Kong, Taiwan, mainland China, Canada and Singapore.
The first case was detected in China's Guangdong province in November 2002 but information about it was not released until February 2003.
On March 12 that year, the WHO issued a global SARS alert. By July, it declared the outbreak contained.
We may not be so lucky next time, say the experts.
"Without substantially more attention and spending, the risks of a catastrophic global disease outbreak keep increasing," warned Olga Jonas, an economic adviser who coordinates the World Bank's response to pandemic threats.
"The annual cost of robust systems for pandemic prevention in developing countries is around $3.4 billion, compared to current spending of less than $450 million. This is woefully inadequate," she told AFP by email.
The odds of a new outbreak multiply each year as more and more globetrotters are exposed to strange new pathogens before going home to ever-more-crowded cities and towns where disease can spread quickly.
"There are viruses out there somewhere, lying quiescent and every now and then they are going to pop up and cause trouble," said Oxford.
"We disturb them... because of the things we do like chopping down trees or whatever."
The corona virus that caused SARS, for example, is believed to have moved from bats, to rats to civets -- once a popular delicacy in southern China.
SARS was followed by the 2009-2010 H1N1 "swine flu" pandemic, which scientists said infected more than a fifth of the world's people and may have killed as many as 250,000 -- about the yearly seasonal flu toll.
Fears about its severity sparked a rush for hundreds of millions of dollars worth of vaccines, soon to be followed by recriminations about the cost of unused drugs and what some critics regarded as an unjustified scare.
It seems not all of SARS' lessons have been taken to heart.
In January, the UN food agency said the world risked a surge in outbreaks of the H5N1 "bird flu" that killed more than 300 people between 2003 and 2011, unless countries strengthen their monitoring of dangerous animal diseases.
A 2012 World Bank report warned a severe pandemic on the scale of the 1918 Spanish Flu that killed about 50-100 million people, may occur once every 100 years and could cost the world $3 trillion today.
"We can never be confident" of handling a new outbreak better, Nuttall said.
"We can only learn the lessons from one outbreak to another. Every time we live through an outbreak we are better prepared."