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 on 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 United States are essentially steady, stuck on a plateau. More than two 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.