The sprawling Camp Ozark in Mount Ida, Arkansas, was shuttered after an undisclosed number of campers and a counselor contracted the coronavirus.
Harvard University students will take courses online, even those living on campus.
In Florida, nearly 50 hospitals on Thursday had intensive care units at full capacity.
The big US reopening of the pandemic summer, it turns out, has gone way off track.
"Basically, we're seeing what happened in New York back in March, except it's happening in multiple metropolitan areas of the country," said Dr. Leana Wen, an emergency physician and public health professor at George Washington University.
"And we don't have the political will and the public willingness to impose the shutdowns as we did back in March."
There were consequences to reopening so soon
The first reported case of Covid-19 in the US was on January 21. By late April, more than one million Americans were infected. At the time, states like Florida were already outlining reopening plans even as New York, an early epicenter, had counted more than 22,000 deaths and 300,000 cases.
A couple of weeks into June, there were two million cases. On Wednesday, less than a month later, the country topped three million cases of the novel coronavirus. This was one day after reporting the highest single day tally so far with 60,000.
More than 90% of the US population was under stay-at-home orders in the early spring as the sheer number of cases brought the healthcare system in some states to the brink.
Still, the Trump administration was eager to get the economy restarted. Protests against shelter-in-place orders erupted from Washington state to western New York. In late May, many states began lifting restrictions despite warnings that expanded testing and contact tracing and better treatment options were needed.
Dr. Anthony Fauci, the nation's leading infectious disease expert, said "divisiveness" and partisanship across the country has contributed to a failure to halt the uncontrollable surge in cases.
"From experience historically ... when you don't have unanimity in an approach to something, you're not as effective in how you handle it," he said Thursday on FiveThirtyEight's weekly Podcast-19.
He added, "When you compare us to other countries, I don't think you can say we're doing great. I mean, we're just not."
Wen said most of the country is now living the consequences of reopening too soon -- and of a failure to have an adequate national coronavirus strategy in place.
"The American people have made tremendous sacrifices to get us to where we are," said Wen, Baltimore's former health commissioner.
"Tens of millions of people have lost their jobs. Kids were out of school and lost valuable time and unfortunately we are in this position where it appears that we have squandered the time that these sacrifices were supposed to buy us."
33 states trend upward in average daily cases
Many states are pausing or rolling back reopening plans. Those accounting for over 40% of the US population have put their reopening on hold, Goldman Sachs reported Thursday. States with another 30% of the population have already reversed parts of their plans.
Washington has largely left reopening plans to the states.
At least 33 states have trended upward in average daily cases -- an increase of at least 10% over the previous week.
CNN medical analyst Dr. Celine Gounder, an internist, infectious disease specialist and epidemiologist, said most states took "halfhearted" steps to combat the spread of the virus, noting, for instance, that stricter shelter-in-place orders could have been implemented earlier than March and April.
"If we had all locked down simultaneously and taken that two-month period to do what we needed to do with preparing and meeting the gating criteria (for reopening) and then all lifted slowly, we'd actually be in a very different place right now," she said.
'A piecemeal approach' and 'mix messaging'
Countries such as South Korea, Germany and Taiwan succeeded in slowing the spread of the virus with steps that included moving quickly to monitor those quarantined and conducting widespread testing and tracing.
"Other countries have been able to combat this because they had a national coordinated strategy instead of a piecemeal approach combined with mixed messaging and even a disdain of science and public health that some of our public officials exhibited," Wen said. "We really did not need to be in this position."
In Florida, heath officials reported on Thursday 8,935 new Covid-19 cases and at least 120 deaths.
Florida, California, Arizona, and Texas account for about 50% of new infections.
Texas on Thursday set another record for highest single-day fatality increases with 105 -- one day after it reported the second highest daily count of new cases at 9,979.
Louisiana, despite progress in recent weeks, has mounting levels of community spread, forcing New Orleans to limit bars and restaurants to 25 patrons inside and prohibit bar seating.
Arizona has led the nation for more a month with the highest seven-day average of new coronavirus cases per 100,000 people, according to a CNN analysis of data from Johns Hopkins University.
Even California, once praised for implementing early restrictions, has seen infection rates rise in Los Angeles to levels not seen since April.
"If the goal is to have schools open in the fall maybe what we should do is to not have bars be open in the summer," Wen said.
A possible 'double whammy' in the fall
The nationwide spikes in cases come less than three months before the start of flu season, which health experts warn could coincide with a new wave of Covid-19.
"We could well face the double whammy come the fall," Wen said.
"So you'll have many patients coming in with the same type of symptoms of fever, shortness of breath, coughs and that will really exhaust our supply of PPE because the patients who come in, you don't know what they have. So you have to treat them as if they have Covid. And it will be a huge strain on our healthcare systems -- for beds, for ventilators, and more importantly, for our healthcare workers."
This is not the year to skip getting a flu shot.
"The health system is going to be overwhelmed with Covid in most of the country this fall and winter," Gounder said.
Fauci and others say there is still time to turn the tide of the pandemic across parts of the South and Southwest.
But states need to start pausing their reopenings, expanding testing and tracing, and encouraging physical distancing and mask wearing.
"I would hope we don't have to resort to shut down," Fauci said Thursday at an event hosted by The Hill.
"I think that would be something that is obviously an extreme. I think it would not be viewed very, very favorably... So rather than think in terms of reverting back down to a complete shutdown, I would think we need to get the states pausing in their opening process."