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The decision to let the soccer game go on between Atalanta and Valencia ion the Champions League in San Siro Stadium, Milan, accelerated the spread of coronavirus says this report in terh WSJ. Practically the whole town of Bergamo moved to Milan to watch the game for a home team. Italian dfoctors say there was a surge in coronavirus cases two weeks after the game on February 19. The virus then hit both Spain and Italy. 

New York also went ahead with games with Madison Square Garden putting 100,000 people through its doors, having not received government guidance to stop. It was not till March 12 that game locations and stadiums were shut down. Doctors in infectious diseases say this was a distinctive amplifier, mass gatherings of people spread the virus quickly, especially one as contagious as this one. This report says in early January the coronavirus arrived from Munich, and in mid-February was still based on rumors about its spread in Lombardy, Italy, so that authorites were lax about it, not understanding the threat being faced.