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Atalanta welcomed Valencia to the San Siro in Milan on Feb. 19 for the biggest fixture in club history, and medical experts now believe the match was one of the principal reasons for the swift spread of COVID-19 in Lombardy, according to the Associated Press' Tales Azzoni and Andrew Dampf.
The northern Italian region has been one of the areas hit hardest by the coronavirus, and with 40,000 Atalanta fans in attendance - or a quarter of Bergamo's entire population - the Champions League last-16 clash is being dubbed "Game Zero" by local media for accelerating the virus' transmission.
"We were mid-February so we didn't have the circumstances of what was happening," Bergamo Mayor Giorgio Gori said this week.
Atalanta were forced to contest the fixture at the San Siro after their home ground was ruled ineligible based on UEFA criteria, and scores of supporters made the trip to Milan.
"If it's true what they're saying that the virus was already circulating in Europe in January, then it's very probable that 40,000 Bergamaschi in the stands of San Siro, all together, exchanged the virus between them," Gori added. "As is possible that so many Bergamaschi that night got together in houses, bars to watch the match and did the same."
As of Tuesday, almost 7,000 people in the Bergamo area have tested positive for the COVID-19 virus and more than 1,000 have died, making it the deadliest province in Italy during the pandemic.
"I'm sure that 40,000 people hugging and kissing each other while standing a centimeter apart - four times, because Atalanta scored four goals - was definitely a huge accelerator for contagion," said Luca Lorini, head of the intensive care unit at the Pope John XXIII hospital in Bergamo.
The Valencia region has confirmed more than 2,600 people infected, and a journalist who traveled to Milan for the match was the second person infected in the Spanish region.
Valencia bowed out of the competition after losing the second leg in front of an empty Mestalla Stadium two weeks later. The club has since confirmed that more than a third of its squad became infected "despite the strict measures adopted" following the match in Milan.
Atalanta have also announced that goalkeeper Marco Sportiello tested positive for the coronavirus.
Italy and Spain are the two European nations most affected by the COVID-19 pandemic. Italy has recorded 74,386 confirmed cases and the world's highest number of casualties with 7,503. Spain has 47,610 positive tests and 3,434 deaths, which is second-highest in the world.