On December 27, a man living in the northern suburbs of Paris showed up to the emergency room with a fever, chest pain, and coughing up blood.

His doctors didn't know it then, but frozen respiratory samples taken during his infection, and tested again months later, would show he had COVID-19, the disease caused by the novel coronavirus, even though he hadn't left France since August.

Back then, the illness was called the Wuhan Coronavirus, a nod to where the new virus was first picked up on epidemiologists' radars, in December.

Nearly a month after the French infection, on January 21, the Centers for Disease Control and Prevention (CDC) would confirm the first known case of the virus in the US: a man in his 30s who'd recently been traveling in China. That same day, the CDC said that while the virus' spread in Asia was concerning, the risk to Americans remained low.

But new evidence suggests the virus was already spreading outside of Asia, not only in European countries including France and Italy, but also perhaps traveling as far as New York and Florida. As 2020 began, the so-called "Wuhan virus" may have been traveling far from Wuhan, China, and beyond the Asian continent for weeks, undetected, and uncontained.

While the US remained laser-focused on spotting the virus hopping a direct flight out of China, it was already busy doing laps around the globe.

President Trump waited to cut off travel to the US from Europe until March 11, months after travelers from that continent might've been escorting the virus across the Atlantic.

There are a few key reasons why it took the US so long to find the coronavirus on its shores. When a new disease emerges, it's easiest to assume (and hope) that the outbreak will remain somebody else's problem, especially when your own disease detection systems and diagnostic tests are sub-par.

Compound those issues with people traveling and at lightning-speed, and a very infectious virus, and you've got a recipe for pandemic disaster.

Our pace of global travel has been unprecedented

The last pandemic was an H1N1 flu that surfaced in Mexico in 2009, amidst a rocky global economy in which there was less international movement.

Global air travel had nearly doubled by the time COVID-19 arrived.

"Public health experts have been saying for years that, first, there's inadequate surveillance around the world of human illness to really detect infections," Marc Lipsitch, a professor of epidemiology at Harvard who studies infectious diseases, told Business Insider in an eerily prescient warning in 2018.

Lipsitch hinted then that one of the best ways for the world to prepare for future outbreaks would be to have more health care professionals stationed around the world, trained to identify and report illnesses before they spread. That kind of defensive disease-fighting strategy would have required massive amounts of cooperation and trust between China, the US, and Europe.

Instead, as January drew to a close, the US decided only to close its doors to foreigners who'd been in China, ignoring the possibility of infections being imported from Europe, and the possibility that more infections may have already been on the ground in the US.

"When the people disperse, it becomes a multi-point outbreak, which is much harder to control," Lipsitch said.

When people move, diseases spread

Epidemic disease outbreaks have always been driven by such human movement.

Indeed, the word epidemic, believed to have been first used by Homer, was originally a way of differentiating "who is back home," as opposed to traveling out of the country.

"We as a society have a terrible tendency to blame the other for infectious diseases," Dr Edward Halperin, the CEO of New York Medical College, told a crowd of healthcare workers earlier this year. He reminded his audience how the arrival of Europeans in the Americas both sickened native people in the US, and in turn brought new venereal diseases back to Europe.

The US made this time-honored mistake once again with the novel coronavirus, by fixating on China and focusing early testing and tracing only on people coming from that country, instead of acknowledging where else the virus may have spread.

"The focus right now is on travelers returning from places where this disease rate is soaring," the CDC's Nancy Messonnier told reporters on February 3, when the city of Wuhan had been locked down for 11 days and the first cases of the virus had been confirmed in Italy, four days earlier.

This is not the first time in recent memory that epidemiologists have obsessed about where a virus was found, rather than thinking about where else it may take root.

Ebola is named for a tributary river in the Democratic Republic of the Congo near where the first known outbreak of that disease occured in 1976. The Middle East Respiratory Syndrome (MERS) was first spotted in Saudi Arabia in 2012, and further back the so-called "Spanish" flu in 1918 probably started in the US, but was later towed back and forth to Europe by American soldiers.

As the US wondered in early 2020 whether the new coronavirus found in China might ever make a home on its shores, the CDC continued only to prioritize testing and diagnosis of COVID-19 for people who'd been either in China, or exposed to someone who had, until February 12.

"The case definition that was established by CDC in the early days was too strict: it required someone to have traveled to China," former CDC epidemic intelligence officer and Emory public health professor Scott McNabb told Business Insider.

"In other words, we were about two weeks behind, the cases were occurring, people were infected, and we were not prepared, because the definition was too strict."

Disease detection is, by its nature, retrospective

Disease surveillance can try to guess where a contagious illness may be on the move with forecasting, but when an unusual cluster of cases of pneumonia is found (that's wht happened in Wuhan in December) clinicians have to press rewind and try to determine who else may have been sick with the same illness after the fact.

"What you do is try and find every case of disease, undiagnosed disease, World Health Organization (WHO) Health Emergencies Program Director Mike Ryan said in a May press conference. "Then you develop a test, find the virus, and you go back and test."

The problem with this kind of investigation is that it only looks at where a virus came from, and that is not that useful in warning us where it's headed next.

Making matters worse, the US CDC is operating with woefully outdated disease surveillance systems, which everyone on the White House Coronavirus Task Force recently agreed are in desperate need of digital upgrades, according to a Washington Post report.

The coronavirus is sneaky, which made it easier to miss than other diseases

Tracking where viral illnesses are coming from, by country or by region, has worked out relatively well for the US in the recent past. The US only ever had eight lab confirmed cases of SARS during that deadly 2002 to 2003 scare, and only treated 11 Ebola patients during the 2014 to 2016 outbreak of that disease.

It's understandable, then, that CDC disease detectives lined up at just a few major US airports in early 2020 to screen people getting off flights from China, checking temperatures, handing out little symptom cards, and instructing travelers about how best to proceed if they got sick when they went home.

But COVID-19 is not like viral illnesses that we've seen before. It is often transmitted when people are asymptomatic, making it trickier to contain than other coronaviruses (like SARS). It doesn't always lead to tell-tale symptoms like vomiting and bleeding, either, making it far less obvious than Ebola. And temperature-checking doesn't really work for this virus.

In fact, New York probably imported most of its first COVID-19 cases from Europe, as researchers at Mount Sinai suggested in one pre-print April study.

"[The coronavirus] came to the New York City area predominately via Europe, through untracked transmissions," Dr Viviana Simon, a Professor of Microbiology and Infectious Diseases at Mount Sinai, said in a press release at the time.

"The virus was likely circulating as early as late-January 2020 in the New York City area. This underscores the urgent need for early and continued broad testing to identify untracked transmission clusters in the community."

Some in the federal government had been warning US health officials that such a scenario might be underway. But if the key federal decision makers were as worried, it did not show, as the US dragged its feet on adequate testing into March, letting the virus quietly spread.

"The idea of epidemiology is that you'd check cases as quickly as you can, so you can investigate those cases, do contact tracing, and stop it - that's what didn't happen," McNabb said.

This article was originally published by Business Insider.

More from Business Insider: