Four states, including Florida and Texas, report highest single-day totals as the U.S. reopens.

More than two months after the United States recorded its worst day of new infections since the start of the coronavirus pandemic, the nation set a record on Wednesday as it reported 36,880 new cases.

The number of infections indicated that the country was not only failing to contain the virus, but also that the caseload was worsening — a path at odds with many other nations that have seen steady declines after an earlier peak. Cases in the United States had been on a downward trajectory after the previous high of 36,739 cases on April 24, but they have roared back in recent weeks.

The resurgence is concentrated largely in the South and West. Florida, Oklahoma, South Carolina and Texas reported their highest single-day totals on Wednesday, but case numbers have been rising in more than 20 states.

The tally of new cases, based on a New York Times database, showed that the outbreak was stronger than ever. The elevated numbers are a result of worsening conditions across much of the country, as well as increased testing — but testing alone does not explain the surge. The percentage of people in Florida who have tested positive for the virus has risen sharply. Increases in hospitalizations also signal the virus’s spread.

Some states, including New York, which at one point had the most daily virus cases, have brought their numbers under control. Hoping to keep it that way, New York — along with Connecticut and New Jersey — said it would institute a quarantine for some out-of-state travelers.

As of Wednesday, more than 2.3 million Americans have been infected and about 122,000 have died.

Gov. Gavin Newsom of California said that his state had recorded more than 7,000 new cases over the previous day.

In Florida, Gov. Ron DeSantis gave no indication that the state would roll back its economic opening, but he urged residents to avoid closed spaces with poor ventilation, crowds and close contact with others.

Mr. DeSantis, a Republican, continued to attribute the rising infections, especially in cities, to younger people who have started to socialize in bars and homes, in spite of rules in many municipalities prohibiting group gatherings. He pressed older people to keep staying home as much as possible, and pleaded with young people to be responsible.

“You need to do your part and make sure that you’re not spreading it to people who are going to be more at risk for this,” he said.

Gov. Roy Cooper of North Carolina announced that the state would pause reopening for three weeks and require face masks. In Texas, more than 4,300 people with the virus are hospitalized, more than double the number at the beginning of June.

The World Health Organization warned on Wednesday that if the Americas were not able to stop the spread of the virus, there may be a need to impose — or reimpose — general lockdowns.

“It is very difficult to take the sting out of this pandemic unless we are able to successfully isolate cases and quarantine contacts,” said Dr. Michael Ryan, the executive director of the W.H.O. health emergencies program. “In the absence of a capacity to do that, then the specter of further lockdowns cannot be excluded.”

He said that the growing number of coronavirus cases in the Americas had not peaked and that the region was likely to see sustained numbers of cases and deaths in the coming weeks.

How the virus stayed a step ahead of the American authorities.

By mid-February, there were only 15 known coronavirus cases in the United States, all with direct links to China.

The patients were isolated. Their contacts were monitored. Travel from China was restricted.

But none of that worked, as some 2,000 hidden infections were already spreading through major cities.

At every crucial moment, American officials were weeks or months behind the reality of the outbreak. Those delays likely cost tens of thousands of lives.

The Times has analyzed travel patterns, hidden infections and genetic data to show how the epidemic spun out of control in the United States.

In other news from around the country:

• The cliffhanger elections on Tuesday in Kentucky and New York were what election officials called a preview of what could happen after the polls close in November: no clear and immediate winner in the presidential race.

  The record number of mailed-in ballots during the pandemic has made vote-counting more unwieldy, and election administrators are straining to deliver timely results.

• The Democratic National Convention will move out of Milwaukee’s professional basketball arena, and state delegations are being urged not to travel to the city because of concerns about the pandemic, party officials said on Wednesday.

• With no major outbreaks among its workers, the U.S. auto industry is nearly back to pre-pandemic production levels, and vehicle sales have perked up more than many industry executives had expected.

• The Walt Disney Company on Wednesday abandoned a plan to reopen its California theme parks on July 17, citing a slower-than-anticipated approval process by state regulators. The announcement came after some employees had criticized the reopening timetable as too fast.

• Travelers to Hawaii can avoid the state’s 14-day quarantine by showing a negative result from a valid coronavirus test, Gov. David Ige of Hawaii announced. The program begins Aug. 1.

A C.D.C. study overlooks an important factor as it measures the effects of pregnancy on Covid-19 patients.

Pregnant women infected with the coronavirus are more likely to be hospitalized, admitted to an intensive care unit and put on a ventilator than are infected women who are not pregnant, according to a new government analysis presented to a federal immunization committee on Wednesday.

Pregnant women are known to be particularly susceptible to other respiratory infections, but the Centers for Disease Control and Prevention has maintained from the start of the pandemic that the virus does not seem to “affect pregnant people differently than others.”

The increased risk for intensive care and mechanical ventilation worried experts. But the new study, by C.D.C. researchers, did not include one pivotal detail: whether pregnant women were hospitalized because of labor and delivery. That may have significantly inflated the numbers, so it is unclear whether the analysis reflects a true increase in the risk of hospitalization.

Admission for delivery represents 25 percent of all hospitalizations in the United States, said Dr. Neel Shah, an assistant professor of obstetrics and gynecology at Harvard University. Even at earlier stages of pregnancy, doctors err on the side of being overly cautious when treating pregnant women — whether they have the coronavirus or not.

The analysis, the largest of its type so far, is based on data from women with confirmed infections of the coronavirus as reported to the C.D.C. by 50 states, as well as New York City and Washington, from Jan. 22 to June 7.

Despite the ambiguities, some experts said that the new data suggested at the very least that pregnant women with the coronavirus should be carefully monitored.

“I think the bottom line is this: These findings suggest that compared to nonpregnant women, pregnant women are more likely to have severe Covid,” said Dr. Denise Jamieson, head of the Covid-19 task force for the American College of Obstetricians and Gynecologists.

Global roundup

In Guatemala and Honduras, the virus has riddled the corridors of power.

Coronavirus contagions have struck at the heart of two Central American governments that are struggling to contain outbreaks in their countries. In one, Guatemala, scores of presidential staff members have fallen ill; in another, Honduras, the pathogen has sickened the president himself.

The condition of President Juan Orlando Hernández of Honduras, who was hospitalized last week and who has pneumonia after testing positive for the coronavirus, was improving after adjustments were made to his treatment this week, according to a statement issued on Wednesday by his office.

Doctors detected a worsening of the pneumonia on Monday, with falling oxygen levels and increasing inflammation, the statement said, but exams on Wednesday showed “a good general condition, without fever, without respiratory difficulty” and with a decrease in inflammation.

In neighboring Guatemala, the number of members of the presidential staff who have tested positive for the virus has climbed to 158, President Alejandro Giammattei said on Wednesday.

The employees work in Mr. Giammattei’s official residential compound in Guatemala City’s historic center, and they include members of his security detail and workers on the compound’s cleaning and kitchen staffs.

Officials first announced the outbreak in early June, when there were a few dozen cases.

Mr. Giammattei said on Wednesday that one of the infected employees, a member of the presidential security service, had died.

The president, however, said that he had been tested three times and that the results had been negative.

In other news from around the world:

• The top U.N. relief official warned on Wednesday of a drastic worsening in the outbreak in war-ravaged Yemen, the Arab world’s poorest country, where he said that 25 percent of those infected die — about five times the global average.

  Many deaths are most likely going unreported, said the official, Mark Lowcock, the under secretary general for humanitarian affairs. But there is one unmistakable measure of the virus’s toll: “Burial prices in some areas have increased by seven times compared to a few months ago,” he said.

• About 4,000 members of a South Korean church who recovered from Covid-19 have agreed to donate their blood plasma for medical research, the church said.

• The Australian airline Qantas will cut roughly a fifth of its work force as it joins other carriers grappling with the near halt in global travel. In addition to the reductions of at least 6,000 jobs, the company will also keep another 15,000 workers on furlough until flying resumes. It will also retire its six Boeing 747 jumbo jets six months ahead of schedule.

• The pilots of a Pakistani airliner that crashed last month in Karachi were busy talking about the coronavirus and repeatedly ignored directions from air traffic controllers before their plane went down, killing 98 people, Pakistan’s aviation minister said on Wednesday.

• The Eiffel Tower in Paris, one of the most visited monuments in the world, reopened on Thursday after a three-month shutdown. Visitors will be allowed only as far as the second floor, and anyone over the age of 11 must wear a face mask.

Demand soars for a steroid that showed promise in treating severe cases, an analysis shows.

Scientists around the world last week cautiously hailed a report that an inexpensive and commonly available steroid had reduced deaths in patients with severe Covid-19. The drug, dexamethasone, is now in high demand, with orders among some U.S. hospitals rising by more than 600 percent in the week after the report, according to an analysis released on Thursday.

In a news conference on Monday, Tedros Adhanom Ghebreyesus, director general of the World Health Organization, said interest in the drug had “surged” after announcements of its “clear benefit.” Dr. Tedros called for a sharp increase in production, while urging continued vigilance about recommended public health measures such as increased testing, contact tracing, physical distancing and hygiene.

The analysis by Vizient, an American health care services company, highlighted dexamethasone’s spike in popularity. Vizient serves more than 5,000 nonprofit health care system members and their affiliates.

Dexamethasone is frequently administered to patients with various conditions that involve excess inflammation, including arthritis, allergic reactions and certain gastrointestinal disorders. The drug, prized for its ability to tamp down certain aspects of the immune system, appears to ease the severity of some of the worst cases of Covid-19. For many infected by the coronavirus, the most severe consequences arise when immune cells and molecules, roused to fight the virus, cannot be kept in check.

Experts caution that dexamethasone is not a cure-all. Patients with milder cases of Covid-19, particularly those not on respiratory support, did not benefit from the drug, the trial’s results showed. And if the steroid is administered too early in an infection, it might even quell the immune system to a degree that compromises a person’s ability to vanquish the virus.

Economists expect 1.3 million new state unemployment claims in the U.S.

With businesses reopening in fits and starts and anxiety increasing over new coronavirus hot spots, the latest unemployment reading on Thursday is likely to offer scant comfort.

Economists surveyed by Bloomberg expect the Labor Department to report that 1.3 million new claims for state unemployment insurance were filed last week, with 20 million people continuing to collect state benefits. If the experts are correct, it would be the 14th week in a row that new claims have topped one million.

The latest data will be published amid conflicting signals for the economy. New York and some other badly affected places are starting to get back to business. But a surge in cases in states that reopened earlier has raised fears of setbacks.

On Tuesday, Gov. Greg Abbott of Texas urged residents to stay home and warned that the state might have to impose new restrictions if the virus could not be contained. And California and Florida have each posted record numbers of new cases in recent days.

Apple shut stores it had reopened in four states — Arizona, Florida, North Carolina and South Carolina — and on Wednesday, the company closed seven stores in Houston.

“The renewed outbreak will hinder the recovery,” said Carl Tannenbaum, chief economist at Northern Trust in Chicago. “I can’t help but think that the willingness of consumers to be in crowded places has diminished. It’s going to be a long haul to get back to where we were before the pandemic.”

The challenges of maintaining a distance.

With eased lockdowns in many places, keeping the recommended distance from others this summer has become more complicated. Here are ideas for handling conflicts over differing ideas of what is safe.

Reporting was contributed by Brooks Barnes, Weiyi Cai, Benedict Carey, Choe Sang-Hun, Reid J. Epstein, Rick Gladstone, James Glanz, Shane Goldmacher, Josh Holder, Apoorva Mandavilli, Salman Masood, Nelson D. Schwartz, Kirk Semple, Mitch Smith, Chris Stanford, Carlos Tejada, Daniel Victor, Derek Watkins, Jeremy Whit, Nic Wirtz and Katherine J. Wu.