The experts have been warning us about it for months, and now there’s evidence to prove it’s happening.

Major outbreaks of Covid-19 coronavirus are occurring all over the country following large gatherings of Americans heedless of any precautions against infection.

They involve college students returning to school; bikers who attended the annual Sturgis rally in South Dakota, a wedding reception in Maine — and even biotechnology executives who gathered earlier this year in Boston.

This last, which took place in late February, is the largest outbreak associated with a single event so far. It now numbers an estimated 20,000 cases across the U.S. and around the world — including Slovakia, Australia and Singapore.

That international conference, hosted by the pharmaceutical company Biogen, is  among the earliest “superspreading” outbreaks yet identified — and it’s been closely tracked and studied by genetic analysts.

For two days they shook hands, kissed cheeks, passed each other the salad tongs at the hotel buffet, never realizing that one among their number carried the coronavirus in their lungs,” reports the Washington Post.

As of Tuesday, the Johns Hopkins Coronavirus Dashboard reported more than 5,750,000 coronavirus cases in the U.S. — and more than 178,000 deaths.

The just -released study of this event and its aftermath is the largest genomic analysis of any U.S. Covid-19  outbreak so far “and is among the most detailed looks at how coronavirus cases exploded in the pandemic’s first wave,” the Post says.

Yet it’s only the largest and most dramatic example of coronavirus spreading identified so far.

A single wedding reception in Millinocket ME, population 4,500 or so, appears to have triggered an outbreak now linked to dozens of cases at a county jail, a nearby nursing home and elsewhere, reports Newsweek

And now, in just the first few days of universities and colleges trying to reopen with in-class teaching, administrators and professors are making no secret of their alarm.

The Universities of Alabama, North Carolina, Missouri, Southern California and  Miami, as well as North Carolina State and others are seeing infections spike, raising the possibility that many students will be sent right back home. Alabama alone has repored more than 500 new cases.

“The rise we’ve seen in recent days is unacceptable, and if unchecked, threatens our ability to complete the rest of the semester on campus,” University of Alabama President Stuart Bell said Monday.

Another potential superspreader event was the annual 10-day Sturgis Motorcycle Rally in South Dakota, which ended Aug. 16, sending hundreds of thousands of bikers out in every direction, cruising across the country to virtually every state.

Scores of cases have already been reported in South Dakota, Minnesota, Nebraska and Wyoming. A study of anonymous cell phone data found that more than 60% of all the counties in the U.S. have already been visited by someone who was at the rally.

The town of Sturgis itself is testing every one of its 7,000 residents. Health officials in at least six nearby states are struggling to track outbreaks linked to the rally, reports the Associated Press. But it may be an impossible task, like tracing everyone who passed through a major travel hub for 10 straight days.

A co-founder of the cell phone-data analysis company Camber Systems compares it to doing contact tracing for the entire city of Washington D.C. — with no idea where anyone was headed.

It all adds up to a very dangerous situation for people all over the place,” he said.