Atalanta fans were walking into a petri dish. In a single mass gathering, they were about to prove how sporting events could end up at the center of a global pandemic.

By then, the coronavirus was spreading through untold numbers of asymptomatic carriers. Forty thousand bouncing, hugging soccer fans were the perfect vector: Experts are now convinced that Atalanta’s 4-1 win over Valencia was a catalyst in turning Lombardy into one of the worst-hit regions on the planet. The coronavirus was so present inside the stadium that night that once Valencia returned to Spain, 35% of its traveling squad eventually tested positive.

“Two weeks after Feb. 19, there was an incredible explosion of cases,” said Dr. Francesco Le Foche, an immunologist in charge of infectious disease at Policlinico Umberto I in Rome. “The match played a huge role in disseminating coronavirus throughout Lombardy and in Bergamo in particular.”