ST. PETERSBURG, Fla. — Florida and other states across the Sunbelt are thinning out the deck chairs, turning over the barstools and rushing to line up more hospital beds as they head into the height of the summer season amid a surge in confirmed cases of the coronavirus.

With newly reported infections running about 40,000 a day in the U.S., Dr. Anthony Fauci, the nation’s top infectious-disease expert, warned on Tuesday that the number could rocket to 100,000 if Americans don’t start following public health recommendations.

Over the past few days, states such as Florida, Arizona, Texas and California have reversed course, closing or otherwise clamping down on bars, shutting beaches, rolling back restaurant capacity, putting limits on crowds at pools, or taking other steps to curb a scourge that may be thriving because of such factors as air conditioning and resistance to wearing masks.

“Any time you have these reopenings, you’re depending on people to do the right things, to follow the rules. I think that’s where the weak spots come in,” said Dr. Cindy Prins, a University of Florida epidemiologist. She warned that things are likely to get worse before they get better.

Hospitals in the new hot spots are already stretched nearly to the limit and are scrambling to add intensive care unit beds for an expected surge in COVID-19 cases in the coming weeks.

Newly confirmed cases in Florida have spiked over the past week, especially in younger people, who may be more likely to survive the virus but can spread it to the Sunshine State’s many vulnerable older residents.

The state reported more than 6,000 new confirmed cases Tuesday. More than 8,000 were recorded on each of three days late last week. Deaths have climbed past 3,500. Floridians ages 15 to 34 now make up 31 percent of all cases, up from 25 percent in early June.

Hospital ICUs are starting to fill up in South Florida, with a steadily increasing number of patients requiring ventilators. Miami’s Baptist Hospital had only six of its 82 ICU beds available, officials said.

In Arizona, hospitals are looking to cram more beds into facilities and hiring out-of-state nurses. State officials have authorized “crisis standards of care” telling hospitals which patients should get a ventilator if there is a shortage.

Dignity Health, which operates several hospitals in the Phoenix area, is converting more areas to treat COVID-19 patients and preparing to put multiple patients in private rooms, spokeswoman Carmelle Malkovich said. It’s bringing nurses from underutilized hospitals in its system to Arizona, and hiring traveling nurses and respiratory therapists throughout July.

Republican Gov. Doug Ducey shut down bars, movie theaters and gyms and banned groups larger than 10 at swimming pools.

Air conditioning could be a factor in hot-weather states where new cases have been spiking, because it recirculates air instead of bringing it in fresh from outside, said Dr. Kristin Englund, an infectious-disease physician at Cleveland Clinic.