As traumatic as the initial wave may have been, the number of people infected didn’t come close to providing so-called herd immunity, experts say. The vast majority still has no natural protection.

“We’re basically at the same place we were at the start of March, in terms of how much of the population is vulnerable,” said Carl Bergstrom, a biology professor at the University of Washington in Seattle.

And people’s willingness to stay home most of the time -- and wear masks when they don’t -- has frayed. For many Republicans, rejecting masks has even become an act of defiance, with President Donald Trump refusing to wear one in public until Saturday, when he visited a military hospital.

“It’s absolute exhaustion,” said Los Angeles Mayor Eric Garcetti, who threatened to reimpose a stay-home order if the outbreak now gripping his city worsens. “Quarantines are the most frustrating things to human beings. Nobody likes being told what to do.”

The states, and their people, may have let their guard down.

Michigan’s two-month shutdown brought infection rates low enough to lift stay-at-home orders by June. Within a few weeks, they started creeping up again.

One of the biggest new outbreaks was at Harper’s Restaurant & Brew Pub, an East Lansing bar near Michigan State University. More than 180 cases have been traced there, according to health officials. Smaller outbreaks have been documented at a bar in Romulus and in Royal Oak, a trendy city north of Detroit.

“Bars and restaurants opened, and that caused a big shift, as well as whatever percentage of people who don’t want to social distance or wear a mask because they think it’s all a hoax,’ said Linda Vail, health officer for Ingham County, which encompasses most of East Lansing.

In response, Governor Gretchen Whitmer banned indoor bar service July 1 and on July 10 made mask-wearing mandatory.

Even though hospitals in Southeast Michigan are better equipped than in March, doctors and nurses are on edge, said Teena Chopra, a doctor who is a professor of infectious disease at Wayne State University in Detroit.