For at least the fourth time in a century, voters will go to the polls amid a lethal viral outbreak, but unlike previous elections held in the shadow of flu, polio and HIV, the novel coronavirus — and the destruction it has unleashed — will almost certainly define the 2020 contest.

From a single case in Snohomish County, Wash., on Jan. 21, the coronavirus has mushroomed in less than 10 months to a widening scourge currently infecting nearly 100,000 Americans a day. As Election Day voters prepared to cast their ballots Tuesday, the medical examiner in El Paso was adding a fourth refrigerated “mobile morgue,” and hospitals in northwest Wisconsin were canceling elective procedures to save beds for patients with covid-19, the disease caused by the virus.

Two-thirds of the public now personally know one of the 9.25 million people who have tested positive for the virus — a new high — polls show. And even more think the worst of the pandemic is yet to come.

“We’ve never had an Election Day in the fog of a pandemic like this,” said Howard Markel, a medical historian at the University of Michigan. “It will, perhaps, be called the pandemic election.”

How those factors affect turnout and results won’t be known until evening, and perhaps not for days or weeks to come. But it is already clear that Tuesday will mark a singular modern-day confluence of a U.S. public health crisis and the election of a president.

“To my knowledge, it’s unprecedented,” said Harvey V. Fineberg, president of the Gordon and Betty Moore Foundation and co-author of “The Epidemic That Never Was,” an analysis of the federal swine flu immunization program in 1976. “Which means one has no basis for comparison.”

In the 1920 presidential election, voters faced a waning threat from the pandemic flu, there was no flu vaccine, and public health was seen as a local issue that did not merit intervention by the president. The Centers for Disease Control and Prevention did not exist. Even during the 1918 off-year election, the pathogen that would eventually kill 675,000 Americans was not a major subject of debate, Markel said.

Periodic flu outbreaks during ensuing decades did not move the political needle much either.

The worst polio outbreaks, in the 1940s and early 1950s, tended to wane as the weather cooled, and the virus was eventually quelled by successful testing of a vaccine in 1955.

Even HIV, which drove activists into the streets, had little impact at election time, at least during the epidemic’s first decade. President Ronald Reagan, who took office in 1981, the year the virus was first recognized, famously would not utter the word “AIDS” until 1987.

Tuesday will be much different.

“I have no idea what it will do in terms of turnout,” said Tom Inglesby, director of the Center for Health Security at the Bloomberg School of Public Health.

“What I’m hoping is that people are not afraid to vote in person if they haven’t voted yet,” he added. “Because I do think it’s possible to vote in a way where you can control your risk so that it wouldn’t be too different from going to the grocery store or going to the pharmacy.”

That includes voting in the late morning or early afternoon, when crowds are smaller, wearing a mask, maintaining distance, and bringing your own pen and hand sanitizer, he said.

Markel, who is also a physician, disagreed. He thinks Election Day turnout will be down, particularly among the elderly, who may conclude that it is too risky to appear at polling places if they haven’t taken advantage of early voting. He said he is unsure how many voters will stay home.

Of course, the virus is widely considered responsible for the record-setting pace of early voting, both by mail and in person, that has preceded Tuesday’s more traditional balloting. Late Monday, the number of early votes cast reached more than 98 million, according to data tracked by The Washington Post.

One of the ways we can counter feelings of anxiety is to control the things we can control,” said Joshua A. Gordon, director of the National Institute of Mental Health. “I think that’s why you’re seeing so much early voting, for example — people are anxious to make sure that the future they’d like to see is going to happen.

“I voted early and it did make me feel a little bit better that I had done my part, the part I can do to forge the future that I want to see.”

The U.S. seven-day average of coronavirus infections, considered the best measure of prevalence, reached 81,740 on Sunday, and nine states set records for hospitalizations, according to data tracked by The Post. More than 230,000 people have died of covid-19.

In Europe and throughout the United States, new restrictions were being imposed. In Maine, where daily counts virtually doubled, Gov. Janet Mills (D) reversed plans to allow bars to reopen Monday and reduced the maximum size of indoor gatherings from 100 people to 50.

“If we do not control this outbreak, we may never get this evil genie back in the bottle,” she said Sunday.

Illinois, which averaged a record 6,367 new cases each day over the past week, expanded restrictions to make a ban on indoor dining effective statewide.

Massachusetts, where the seven-day average of new cases stood at nearly 1,300 on Sunday compared with 689 two weeks earlier, issued a new advisory directing residents to stay home between 10 p.m. and 5 a.m., except for necessary activities. Gov. Charlie Baker (R) emphasized that he was not shutting down the economy or schools.

“I think what we’re trying to say here is by 10 o’clock, people should use their heads and be with the people they live with instead of continuing to perpetuate this constant churning of folks,” Baker said.

On Saturday, five Mayo Clinic hospitals in hard-hit northwest Wisconsin began postponing elective procedures such as colonoscopies and routine physicals to free beds for the rapidly increasing number of covid-19 patients in the area around Eau Claire. The move probably will last throughout November, said Richard A. Helmers, regional vice president for Mayo’s health system there.

“Simply put, if we cannot slow the rate of infection, we risk overwhelming our health-care system,” Jason Craig, regional chair of administration for Mayo, said at a media briefing Friday. About 230 staff members are off work because they have contracted or been exposed to the coronavirus in the community, he said.

Several European nations reimposed nationwide shutdowns — most recently Germany, where restaurants, bars and recreational facilities closed Monday in a modified, less strict version of spring’s lockdown.

But in parts of the United States, Halloween weekend partying was not halted. In Utah, authorities said several thousand people attended a large, rave-like gathering with multiple DJs on Saturday night that was discovered when a woman was knocked unconscious while crowd-surfing, according to KSL.com. Police also shut down numerous parties in the college town of Boulder, Colo., over the weekend, Denver’s KMGH-TV reported.

On Tuesday, less than a year after the virus emerged from Wuhan, China, to circle the globe, a jittery electorate will settle the question of who will lead the next phase of the U.S. pandemic.

“With all that is going on right now, many of us are feeling quite anxious about our current situation, and our future,” Gordon said. “That is not surprising given the degree of uncertainty there is about the pandemic, about the election, about the controversies over race in America, about climate — about all these things. Uncertainties breed anxiety.”