LOS ANGELES >> The seeds of the latest surge in coronavirus cases in California appear to have been planted around Memorial Day.

People had been pent up in their homes; businesses shuttered for months amid the stay-at-home order began to open. And as the reopening accelerated, a lot of people were ready to get out.

The beckon of summer rituals followed — day trips to the beach, Memorial Day barbecues, graduation celebrations, Father’s Day gatherings. Around the same time, historic protests began, triggered by outrage over the death in Minneapolis of George Floyd while in police custody, which sparked unprecedented demonstrations across the nation, including in the streets of California.

It would take a few weeks of incubation. But it’s now clear that Memorial Day was the beginning of something. A Los Angeles Times analysis has found that new coronavirus hospitalizations in California began accelerating around June 15 at a rate not seen since early April.

Statewide, the daily number of people in hospitals with a confirmed infection of the coronavirus has jumped nearly 50% from when it had been stable in mid-April, the Times found. As of Saturday, there were 4,498 people hospitalized; in mid-April, the daily average was about 3,100.

It can take two weeks for the virus to incubate in the body, and an additional week or two after that to result in the hospitalization of severely ill people. That means more people may have gotten exposed to the virus around the week of Memorial Day or shortly thereafter, said Dr. Robert Kim-Farley, a medical epidemiologist and infectious-diseases expert at the UCLA Fielding School of Public Health.

Health officials and experts always knew that easing stay-at-home orders would result in a higher level of cases. But they also said they had to make progress in reopening.

Continuing to keep society shut down at such an extreme state for too long causes its own ill effects, whether it be more homelessness and deaths due to greater poverty or the effects of denying schoolchildren their in-person education, Kim-Farley said.

“It’s a luxury to shelter in place,” added Dr. Kirsten Bibbins-Domingo, chair of the University of California, San Francisco’s Department of Epidemiology and Biostatistics. “We have to think about how we open and minimize risk. We’re going to be living with this virus for a long time.”

But there is no textbook to figure out how to reopen California safely amid the world’s worst pandemic in a century, faced with a never-before-seen coronavirus.

The only way to figure out how to open is to do it gradually and dial things back if the disease spreads so fast it might overwhelm hospitals later. And that’s what’s happening now, Kim-Farley said.

“Now, we’re recognizing things are going up. So we’re dialing it back down again,” Kim-Farley said.

A Times analysis found that as of Sunday, 5.9% of coronavirus test results received over the last week are positive. That’s a significant jump from the figure the previous week, when it was 4.8%.

The rate is even worse in Los Angeles County: As of Sunday, the seven-day average of coronavirus tests being confirmed as positive was 8.7%. Two weeks ago, it was 5.8%.

Across the state, public health officials have expressed alarm at the rising rate of cases and hospitalizations. On Sunday, Gov. Gavin Newsom ordered Los Angeles County and four counties in the San Joaquin Valley — Fresno, Kern, Kings and Tulare — to shut down bars. Imperial and San Joaquin counties, which have not allowed bars to reopen, are being ordered by the state to keep them closed.

The state also recommended that the following counties issue orders closing bars: Riverside, Sacramento, San Bernardino, Santa Barbara, Stanislaus and Ventura. State officials are recommending that two Bay Area counties, Contra Costa and Santa Clara, keep bars closed.

State officials say the consumption of alcohol in bars impairs judgment and leads to decreased use of face coverings and keeping socially distant from other people. The spaces are also loud, often requiring people to speak in louder voices, which can lead to the spray of potentially infectious oral droplets while talking.

It was on June 12 that Newsom allowed counties to decide when to allow bars to reopen; L.A. County gave the green light June 19.

As of Sunday, 16 California counties have seen increases in hospitalizations that have exceeded 10% in the last three days, according to The Times’ California coronavirus tracker. They affect the state’s most populated regions: Southern California, the San Francisco Bay Area, San Diego County and the Central Valley.

“While we did anticipate increases in cases as sectors reopened, we did not expect the increases to be this steep this quickly. Without immediate actions to slow the spread, we risk having too many people requiring hospital care and possibly overwhelming our health care system,” Barbara Ferrer, L.A. County’s public health director, said Friday.

In a stark warning, Ferrer said Sunday that a failure to follow pandemic safety orders puts the public at risk. “Otherwise, we are quickly moving toward overwhelming our health care system and seeing even more devastating illness and death,” she said.

More than 5,900 people infected with the coronavirus have died in California, with more than 3,300 of them in L.A. County.

Officials and experts have pleaded with the public to wear masks. Dr. Scott Gottlieb, who headed the U.S. Food and Drug Administration as an appointee of President Donald Trump until last year, tweeted that the chief measure to save lives, allow schools to reopen in the fall and keep businesses operating safely is “universal masking.”

A Gallup poll found that 84% of U.S. adults said they’d worn a face mask outside of their home in the past week. But there are some people who are vociferously opposed to complying with California’s mask-wearing order.

A Los Angeles taco chain said Sunday it was forced to temporarily close two locations after a mounting onslaught of harassment from customers angered by the business’ “no mask, no service” policy.

The harassment, including racial epithets and drinks being hurled at workers through order windows, has taken an emotional toll on the mostly Latino employees, Hugo’s Tacos part-owner Bill Kohne told the Times.

L.A. County officials warned about the “appearance of phony mask exemption cards that depict a government seal with threatening information.” People who have a medical condition that prevents the use of a mask should wear a face shield instead, county officials said.

Kim-Farley, a former senior health official with L.A. County and the U.S. Centers for Disease Control and Prevention, said there are three protagonists — individuals, businesses and county governments — who each need to do their part to limit the spread of the disease.

Not only do government officials need to analyze data for worrisome trends, but individuals must wear face coverings and stay 6 feet away from other people, and business owners need to keep their establishments from getting crowded and regularly disinfect high-touch surfaces.

It’s possible too many Californians responded to the reopenings of businesses as a license to resume life as they did before the pandemic arrived, Kim-Farley said. Californians never endured the trauma New Yorkers did of seeing their hospital system get overloaded by COVID-19 patients in the spring.

“It may be that they’re no longer as conscious about masking and physical distancing,” Kim-Farley said.

Another factor is that the political discord in the U.S. and California over the response to the pandemic, such as political fights over using face masks, is hurting our ability to control the epidemic, experts say. Countries that have had a unified public response to broad pandemic control measures, like New Zealand and Taiwan, have kept the virus from spiraling out of control, experts say.

The pandemic could have triggered the sense of unifying around a common enemy in the U.S. “Unfortunately, we’ve made it such that it’s become very divisive and become very politicized,” Kim-Farley said.

“If everyone could be very responsible about practicing the masking and physical distancing, we should be able to open up without having, again, the surges we’re seeing.”

The reverberations of severe outbreaks in Southern California are now being felt across the state.

The San Francisco Bay Area is being confronted with a large outbreak of COVID-19 at San Quentin State Prison, where the virus was imported after 121 prisoners were transferred from the California Institution for Men in Chino without being tested for the virus. On Sunday, there were 832 active cases of COVID-19 at San Quentin, Marin County officials said.

More than 75 staff at the prison have been infected, and some of them live in Marin County, health officials said. San Francisco hospitals are taking in patients not only from San Quentin, but also from Imperial County, which Newsom on Friday recommended implement a new stay-at-home order.

San Francisco has seen its number of cases spike, with the seven-day average now worse than at any previous point in the pandemic.

“We talked about flattening that curve. That curve is not flat right now. In fact, that curve is getting more and more vertical,” Dr. Grant Colfax, the San Francisco director of public health, said Friday.

“We hope that if everyone does their part to cover their face, social distance and take good hygiene measures that that curve will start going back down and flatten, and we will be able to move forward. But it will take time,” Colfax said.

San Francisco has halted the planned reopening of outdoor bars, hair and nail salons, tattoo shops, museums, zoos and outdoor swimming that was expected to take place Monday.

So far, the number of daily deaths statewide has not jumped. At its highest, the seven-day average of daily fatalities was 79 on May 21; on Friday, it was about 64 deaths a day.

That may be because the people now getting infected are increasingly younger adults, who are less likely to suffer severe complications than older people, or because it will simply take more weeks before those now being hospitalized will die.

Newsom has warned that he expects the daily death toll to rise in the coming weeks.

Experts say new coronavirus infections have also reached their highest levels nationally since the start of the pandemic.

“There is explosive growth of COVID-19 epidemic across the Southeast and Southwest of the United States, with certain exceptions. And we’re in fact seeing a new rise to the highest levels we’ve ever seen in the United States,” Dr. George Rutherford, a UC San Francisco epidemiologist and infectious-diseases expert, said at a campus town hall Friday.

Nationally, there are a cumulative 2.5 million coronavirus cases and more than 125,000 deaths, according to Johns Hopkins University.