Cordoba, Spain CNN  — 

Unseasonably warm weather, Champions League football and other major events, homes on the beach and the café culture: just a few of the factors that may have helped carry an insidious virus across southern Europe – from country to country and city to city, from Italy to Spain and Portugal.

The virulence and lethality of the coronavirus was compounded by the fact that symptoms take at least several days to emerge – and by a series of missteps by governments as they chased its impact, before eventually succumbing to the reality that only a total lockdown could stem the tide.

More than 5,000 people have died since a coronavirus outbreak exploded in Spain, one of the countries worst affected by the pandemic. The country has more than 54,000 active cases of the virus, according to recent figures from the ministry of health.

On February 19, nearly 3,000 Valencia football fans traveled from Spain to Milan to watch their team play Atalanta in a European Champions League game. Some 40,000 Italians were also at the game, many of them from Bergamo and surrounding towns.

Atalanta fans cheer during a Champions League match between their team and Valencia on February 19 in Milan.

Milan was buzzing that evening, according to the Mayor of Bergamo, Giorgio Gori. Besides those who attended the game, “others watched it from their homes, in families, in groups, at the bar,” Gori said this week. “It is clear on that evening there was an opportunity for a strong spread of the virus.”

Immunologist Francesco Le Foche is of the same view, telling the Corriere dello Sport: “It’s probable that there were several major triggers and catalysts for the diffusion of the virus, but the Atalanta-Valencia game could very well have been one of them.

“With hindsight, it was madness to play with a crowd present, but at the time things weren’t clear enough,” Le Foche said.

Two days later, in the town of Codogno, some 60 kilometers (40 miles) from Bergamo, a 38-year-old man known as “patient one” was diagnosed with the virus. But by then, according to research reported by nearly 20 Italian specialists, the virus had long been on the move.

“At the time of detection of the first Covid-19 case, the epidemic had already spread in most municipalities of southern Lombardy,” they say.

After examining nearly 6,000 confirmed cases the researchers discovered that there were already 388 patients in the Lombardy region with novel coronavirus as early as February 19 – even if they’d not yet been identified as such.

“In the week that followed, [the] Codogno area, as well as several neighboring towns in southern Lombardy, experienced a very rapid increase in the number of detected cases,” the researchers note.

Although the research has not been published in a peer-reviewed journal, it was mentioned in a report by the journal Nature on Friday.

By then the visitors from Valencia were back home. One of them was sports journalist Kike Mateu. He told CNN that four days after returning home he had a cough and difficulty breathing. A few days later, “I decided to go to the hospital because by then there was an explosion of coronavirus cases in Lombardy.”

People keep their distance from each other as they wait in a supermarket queue in Barcelona on March 16.