“To fight an enemy you have to better understand it,” Cohen tells Fortune. “All doctors should do the same studies to help us understand the life of the virus.

Courtesy of Yves Cohen

Cohen, who heads the intensive-care unit at Avicenne Hospital in the northeast Paris suburb of Bobigny, decided with colleagues in April to retest the nasal swabs and reexamine the chest X-rays of 14 patients they had treated last December; all the patients had been suffering from various respiratory illnesses.

Among them was Amirouche Hammar, a 42-year-old Algerian-French fishmonger living in the nearby suburb of Bondy, who arrived at the emergency room last December 27, coughing blood and struggling to breathe. Four months later, his nasal swab, which still sat in the hospital lab, showed that Hammar had been suffering from COVID-19—a disease almost no one in Europe and the U.S. had even heard of in December and for which there was no test available.

It is unclear how Hammar was infected. Fully recovered, he is back home, kicking a soccer ball around his back yard and somewhat shocked at his sudden “patient zero” celebrity, as well as the fact he apparently survived the worst pandemic in a century.

Michel Euler—AP Images

Cohen says that since March, when he first heard that the outbreak had likely begun in China as early as November, he suspected that some infected people had surely made their way to France, which has extensive business links with China and receives about two million Chinese tourists a year. Until his findings were published in the International Journal of Microbial Agents, health authorities believed the first case outside China appeared on January 24 in the French Alps, carried to Europe by a British man who flew from Singapore. Many officials have speculated that the virus then rampaged across Europe as people returned home from Alpine ski vacations.

But Cohen and his colleagues doubted that theory.

“We thought surely there were patients of COVID-19 in December,” Cohen says. “All day long, airplanes were coming from China.” The hospital and Hammar’s home are both relatively close to Charles de Gaulle Airport.

Having stumbled on a key clue to the pandemic, Cohen now believes health authorities across the world have likely counted a minuscule fraction of COVID-19 infections. About 3.77 million people are counted as infected, and in France, there have been 174,224 confirmed cases, and about 25,812 deaths, according to Johns Hopkins University. 

Cohen believes those figures are a tiny fraction of the reality, reflecting how the virus has spread at lightning pace. Based on the small number of tests available, he estimates that in France alone—with a population of 67 million—the true number of COVID-19 cases is between three and five million people.

In other countries, too, Cohen believes, “doctors will eventually find millions more cases.”

So far, there are just a few attempts to do so—but those suggest Cohen might be right. After New York State health authorities conducted antibody tests on a random sampling of people, they concluded that about 2.7 million people in the state have probably been infected with the virus, about 10 times the official rate.

Epidemiologists say that looking for missing cases, as Cohen did, is crucial in understanding the pandemic and preparing for the next one.

“The incredible speed with which it started and spread, should be investigated,” Tom Jefferson, a clinical epidemiologist based in Rome, who is part of Oxford University’s COVID-19 Evidence Service, told journalists on Thursday in a Zoom meeting hosted by the Global Investigative Journalism Network. “We should really compile a universal global history of this pandemic.”

That could take many years to do. And Cohen says that by then COVID-19 cases will probably have spiked again.

Cohen says he and other doctors are bracing themselves for a fresh surge of infections in the coming weeks, as countries—including France—reopen and people begin venturing out of their homes again. France, which has been under a severe lockdown since March 17, has declared a staggered reopening beginning on Monday, dividing the country into hard-hit red zones and relatively unaffected green zones. The French will no longer be required to show police signed documents stating why they have left their house, as they have done for the past seven weeks.

Paris, in the red zone, will not return fully to normal until later in June. Only elementary schools will reopen next week. But already, thousands of Parisians have begun spending more time outdoors.

Cohen predicts his hospital will again be overwhelmed with COVID-19 patients. “Before the crisis, we had 16 ICU beds. In April we had 48 beds in the ICU, and now we have 32.”  

He predicts that number will rise sharply after France reopens. His final word of advice: Do not grow complacent, as life begins again after months of restrictions. He worries particularly about the current focus on facial masks.

“I am afraid people will think that with masks, everything is good,” he says. But as crowds increase, so will the risks. “There will be many, many people in the parks, on public transportation, on trams,” he says. “We will see a new peak of the epidemic.”

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