CHICAGO – The warning has echoed ominously for weeks from epidemiologists, small-town mayors and county health officials: Once states begin to reopen, a surge in coronavirus cases will follow.

That scenario is now playing out in states across the country, particularly in the Sun Belt and the West, as thousands of Americans have been sickened by the virus in new and alarming outbreaks.

Hospitals in Arizona have been urged to activate emergency plans to cope with a flood of coronavirus patients. On Saturday, Florida saw its largest single-day count of cases since the pandemic began. Oregon has failed to contain the spread of the virus in many places, leading the governor Thursday to pause what had been a gradual reopening.

And in Texas, cases are rising swiftly around the largest cities, including Houston, San Antonio and Dallas.

"I'm very concerned about it," said Mayor Eric Johnson of Dallas, noting that after months of warnings and isolation, many residents had stopped wearing masks and maintaining social distance out of sheer fatigue. "They've been asked for quite some time to not be around people they love and that they want to spend time with. Wearing a mask is not pleasant. And I think people are tired."

For close to a month, much of the United States has looked like a nation open or beginning to open, and increasingly unfettered by restrictions meant to slow the spread of the coronavirus. With many government limits removed and people left to make individual choices about precautions, Americans have gone back to salons and restaurants, crowded into public parks and, in dozens of cities, joined large public demonstrations protesting police misconduct.

Overall, daily coronavirus cases across the U.S. are essentially steady, stuck on a plateau. More than 2 million people have now contracted the virus in this country, according to a New York Times database, and every day, about 21,100 new known cases are reported, not much lower than the numbers from a month ago. About 800 people die from it each day. Those figures have both dropped significantly since peaking in April.

But as of Saturday, the daily number of new coronavirus cases was climbing in 22 states, shifting course from what had been downward trajectories in many of those places.

The spikes in cases bring leaders in these states to a new crossroads: Accept the continued rise in infections as an expected cost of reopening economies or consider slowing the lifting of restrictions aimed at stopping the spread or even imposing a new set of limits.

In Houston on Thursday, the county's top elected official warned that the community was "on the precipice of a disaster" and urged residents to minimize contact with others. More than 300 new cases have been identified in that county on each recent weekday.

But at the city's Galleria mall, there were few signs of concern: People stood in a tightly spaced line for pretzels at an Auntie Anne's kiosk. At California Nails, two women sat maskless during pedicures. Signs urged social distancing, but in crowded walkways outside stores, shoppers brushed past one another only inches apart.

Throughout most of Florida, the reopening of public life has allowed bars and movie theaters to operate at half capacity and gyms at full capacity. On June 5, the state loosened restrictions further, even as the caseload was beginning to go up.

In Salt Lake City, some people are now behaving as they did before the coronavirus pandemic, even amid a rise in cases, said Teresa Kehl, a Utah resident who runs summer soccer camps with her husband.

"We went to a restaurant the other night and none of the employees had masks on," Kehl said. "It was kind of shocking."

Dr. Angela Dunn, the Utah state epidemiologist, has traced the state's resurgence in the coronavirus to the state's reopenings, which began before Memorial Day.

"The timing directly correlates with our loosening up restrictions," Dunn said. "That definitely has something to do with it."

As testing capacity has increased, so has the number of cases being counted, and officials in places like Arizona and Florida say the increase in cases may be explained, at least partly, by the growing availability of tests.

But epidemiologists said that even taking into account a rise in testing, the increase in confirmed cases in Sun Belt states suggested increased transmissions. Other measures, such as the percentage of positive tests and hospitalizations, reflect that worsening outlook. In Florida more than 4.5% of those who tested between May 31 and June 6 had the virus, compared with about 2.3% of people who sought tests in mid-May. Earlier in the pandemic, the percentage of people testing positive in Florida was higher, but that was during a period when testing was far more limited. Similar rates in Arizona and Texas have also risen in recent weeks.

For states with growing coronavirus outbreaks, some officials have arrived at the same conclusion: The rise in infections is unfortunate but inevitable.

"We are not going to be able to stop the spread," said Dr. Cara Christ, the Arizona state health director. "And so we can't stop living as well."