JONATHAN EPSTEIN: Theres really two fundamental questions that we ask when were looking at epidemics. How did this happen? And how can we prevent it from happening again?

ARCHIVAL (PBS NEWSHOUR, 3-20-20):JUDY WOODRUFF: More of the country is closing down tonight.

ARCHIVAL (CBS, CBS THIS MORNING, 10-22-20):NEWS REPORT: The coronavirus crisis is escalating across America.

ARCHIVAL (ABC NEWS, 11-2-20):NEWS REPORT: The number of people dying from the disease is rising in 26 states.

ARCHIVAL (CNN, 3-3-20):NEWS REPORT: Hospitals across the country are reaching a breaking point.

JONATHAN EPSTEIN: Weve known now for a long time that the single biggest driver of epidemics are people. Its things that were doing that change the environment around us that bring us into closer contact with wildlife. Weve known that coronaviruses were a risk for emerging. If you dont understand where these viruses come from originally, were just going to have this keep happening again.

TEXT ON SCREEN: Hanoi, Vietnam

NARRATION: In February 2003, an American businessman was taken to a hospital in Hanoi with what appeared to be a bad case of the flu. But his chest x-ray looked unusual to Dr. Olivier Cattin.

DR. OLIVIER CATTIN: It looks like it was snowing in their lungs. We got some patchy opacities everywhere. And I say, Whatsits new. Ive never seen that in my textbook.

NARRATION: The patient appeared to be recovering at first, but then his oxygen levels crashed, leaving him in critical condition, unable to breath without a ventilator.

OLIVIER CATTIN: The lungs turned completely white, completely white. You cannot see anything. It was white lungs.

NARRATION: The patient was medevacked to Hong Kong, where he died. By then, Dr. Cattin had gotten some disturbing news: four nurses were sick.

OLIVIER CATTIN: So we start to retest everything, influenza, chest x-ray. The first day everything was normal. On the Thursday morning, there were seven people sick. On Friday more than 20 people sick. All of the staff were deteriorating. The chest x-rays were changing, worsening, and one of the nurses start to have white lungs. And we say, Okay. We, we have this feeling we all are going to die.

NARRATION: A coworker had alerted the World Health Organization, where Dr. David Heymann had already heard reports of an unusual illness spreading in China. The outbreak in Vietnam helped convince him there might be a new disease spreading around the world.

DR. DAVID HEYMANN (FORMER ASSISTANT DIRECTOR-GENERAL, WORLD HEALTH ORGANIZATION): By Friday of that week, there were reports from Singapore. There were reports from Hong Kong and also from Canada. We thought it was very urgent that this infection be stopped if it could be stopped.

ARCHIVAL (NBC, 3-15-03):DR. DAVID HEYMANN: Today, the World Health Organization has announced a global emergency because of this disease, because of the speed with which it is spreading.

DAVID HEYMANN: We gave it a name. We called it Severe Acute Respiratory Syndrome.

ARCHIVAL (CBS, 4-8-03):NEWS REPORT: The disease, known as SARS, has now spread to at least 17 countries.

ARCHIVAL (NBC, 3-31-03):TOM BROKAW: Tonight, at least 60 people are dead because of it.

NARRATION: As cases continued to spread, scientists were starting to understand the new disease.

ARCHIVAL (CBS, 4-8-03):NEWS REPORT: Scientists have been zeroing in on something called the coronavirus.

NARRATION: Though coronaviruses typically cause just a mild cold, this one turned deadly. But where had it come from? SARS was traced back to a handful of early cases people who were linked to wildlife markets in southern China.

JONATHAN EPSTEIN (VETERINARIAN AND EPIDEMIOLOGIST, ECOHEALTH ALLIANCE): There are literally cages and cages full of wild animals, caught from the wild, stacked on top of each other. You have people handling these animals, butchering them live in the market and possibly touching their face, having a cigarette or eating. And so, theres a lot of opportunity for people to get exposed to animal viruses.

NARRATION: Scientists suspect that it was at those markets that the virus jumped species.

ARCHIVAL (CBS, 5-23-03):NEWS REPORT: Scientists now believe these strange looking creatures called civet cats may be the source of the SARS virus in humans.

NARRATION: The Chinese government banned the sale of wild animals and killed thousands of civet cats. And a combination of quarantines, travel restrictions and luck eventually stopped SARS before it spread out of control. But it was a dire warning of what could happen.

JONATHAN EPSTEIN: SARS coronavirus was really the first pandemic of the 21st century. It infected people around the world in more than 25 countries.

DAVID HEYMANN: We saw the emergence of a new coronavirus thought to be from an animal. That was a wakeup call that things could spread rapidly around the world.

NARRATION: It was the kind of scenario that many public health workers had been warning about for years.

ARCHIVAL (CBS, 12-1-76):NEWS REPORT: Until two months ago, doctors didnt even know this virus existed.

NARRATION: Decades before SARS, public health investigators started to notice a troubling trend: New viruses were appearing. The first Ebola outbreak in 1976 killed nearly 300 people. In the 1980s, HIV spread around the world. Its thought to have come from chimpanzees. In 1997, a new strain of flu jumped from chickens to people in Hong Kong.

JONATHAN EPSTEIN: What weve seen now is an acceleration of emerging infectious diseases. There have been more and more outbreaks over the past few decades.

NARRATION: Its estimated that up to 75 percent of those new, or emerging, diseases come from animals and spillover into humans. Jonathan Epstein has spent years researching how that process happens, and an outbreak of a virus called Nipah in Malaysia offered new clues.

JONATHAN EPSTEIN: This outbreak started in people when farmers were getting exposed to sick pigs. This was a new virus that hadnt been seen before, and it was killing about 40 percent of the people that got it.

NARRATION: The Malaysian government slaughtered about a million pigs, but the virus didnt go away.

Follow-up research found that the pigs contracted Nipah from bats that live in the rainforest where the pig farms were built.

ARCHIVAL (CBS NEWS, 60 MINUTES, 2004):SCOTT PELLEY: This is where the first infections happened. Notice the fruit trees over the pig pens.

NARRATION: The bats were attracted to mango orchards on the farm.

JONATHAN EPSTEIN: Though normally these bats would be foraging on figs or other wild fruit in the forest, they were drawn to these large orchards. They would eat mangos, and, in the branches that were overhanging the pig enclosure, they would drop pieces of fruit.

NARRATION: That outbreak was the basis for the movie Contagion. And other outbreaks have cropped up since then often because people or livestock encroached on wild animals.

JONATHAN EPSTEIN: The human population, as it grows and grows, is putting incredible pressure on natural systems all over the world in ways that we never did historically.

DENNIS CARROLL (FORMER DIRECTOR, EMERGING PANDEMIC THREATS, USAID): There are eight billion people living on this planet. Thats six billion people more than we had at the time of the 1918 pandemic. And that dramatic increase has resulted in a significant disruption of the ecosystems around us.

NARRATION: Most scientists believe the interaction between people and wild animals led to the COVID-19 pandemicand they say the pandemic shows we havent learned the lessons from previous outbreaks. Especially from SARS.

After the SARS outbreak, there was a flurry of research into treatments for coronaviruses and into discovering where they come from.

JONATHAN EPSTEIN: The hope was, we would really continue to invest in understanding how viruses like SARS emerged. But over time, people forgot about the urgency of SARS. You know, resources started to shift elsewhere to other crises or other emergencies.

DAVID HEYMANN: Funding dried up after the outbreak was contained and so did the research that should have gone on that could have possibly prevented future outbreaks.

NARRATION: Epstein was part of a team that did continue to research SARS after the outbreak was over. It turned out the SARS virus like many other diseases he studied most likely came from bats.

In fact, by 2017, Epsteins colleagues had discovered dozens of coronaviruses in bats, including one 96 percent similar to the virus that causes COVID-19. And they found evidence that some of those viruses had already made the jump into people.

JONATHAN EPSTEIN: Knowing that people and bats were having contact gave us a strong signal that there was a risk of another coronavirus emerging. And, the question was really going to be which one would it be and when would it happen.

NARRATION: Though its not certain how the new coronavirus emerged, it likely originated in bats and later passed through one of Chinas wild animal markets. They were allowed to reopen after the SARS outbreak ended.

JONATHAN EPSTEIN: Weve known that coronaviruses like this were a risk for emerging for a long time. This is really, if it isnt our final warning, it certainly should be a wakeup call right now that we need to change the way were doing things. There may be a virus out there that is more transmissible and even more lethal, and we cant afford to wait until that makes its way out of an animal and into people.

NARRATION: Its impossible to prevent spillovers entirely, but there are ways to reduce the risk regulating or shutting down the trade in wildlife, early detection of new diseases, and separating domestic and wild animals.

Dennis Carroll started a project called PREDICT to try to find unknown animal diseases.

DENNIS CARROLL: How many different viruses are there? PREDICT allowed us to begin quantifying that. There are about 1.6 million different viruses. We estimated about 600,000 of them had the potential to infect people.

NARRATION: That doesnt mean theyre likely to turn into a pandemic. And while the PREDICT project ended, similar research could identify high risk-areas and patterns in how viruses emerge. But stopping the next pandemic would require a major shift in resources and perspective

DAVID HEYMANN: Theres a whole way of looking at this that we must develop. We need to begin working in a new way having the animal and human public health communities working much more closely together.

DENNIS CARROLL: Lets change the paradigm. Stop reacting. Go out. Know the viruses before they know us. Stop them before they have a chance to infect us.