Barbara Alexander’s Christmas tradition is to drive 2½ hours to the 40-acre farm her parents bought seven decades ago in southeastern North Carolina. It’s a big affair: 35 family members arrive by Christmas Eve.

This year, she is thinking: wait till next year. She’ll stay home in Durham, N.C., with her husband, teenage son and 96-year-old mother, Marble Dudley. As a physician and the president-elect of the Infectious Diseases Society of America, Alexander is fully aware of the risks of holiday gatherings in the middle of a pandemic and the vulnerability of her nonagenarian mother.

“Covid doesn’t care that it’s a holiday, and unfortunately covid is on the rise across the nation,” she said. “Now is not the time to let our guard down and say it’s the holiday and let’s be merry. I think we need to maintain our vigilance here.”

The coronavirus pandemic numbers have been going the wrong direction for more than a month, topping 80,000 newly confirmed infections daily across the country, with hospitalizations rising in more than three dozen states and deaths creeping upward. Now, the United States is barreling toward another inflection point: a holiday season dictated by the calendar and demanded by tradition.

The anticipated surge in interstate travel, family gatherings and indoor socializing is expected to facilitate the spread of covid-19, the disease caused by the novel coronavirus. This isn’t like the run-up to Memorial Day or Independence Day: Barbecues outdoors, or pool parties, aren’t on the itinerary of many people.

The fall and winter holidays are homey by nature. Respiratory viruses thrive in dry, warm indoor conditions in which people crowd together. The statistical peak of flu season typically comes close on the heels of Christmas and New Year’s. Colder weather is already driving people indoors.

The government’s top doctors have said they believe the recent national spike in infections has largely been driven by household transmission. Superspreader events have gotten a lot of attention, but it’s the prosaic meals with family and friends that are driving up caseloads.

This trend presents people with difficult individual choices — and those choices carry societal consequences. Epidemiologists look at the broad effect of a contagion, not simply the effects on individuals. Thanksgiving, for example, is an extremely busy travel period in America. The coronavirus exploits travelers to spread in places where it has been sparse or absent.

“I am nervous about Thanksgiving,” said Andrew Noymer, an epidemiologist at the University of California at Irvine. “I’m nervous because I know what happens when you multiply the risks by millions of households.”

The scientists are not telling people to cancel their holiday plans, necessarily. But they are urging people to think of alternative ways to celebrate. They do not say it explicitly, but they are encouraging a kind of rationing of togetherness.

“This is not the cold. This is not the flu. This is much worse. People are dying. Our well-being as a country depends on us getting this thing under control,” Alexander said.

Public-health officials doubt an elegant way exists to finesse the 2020 pandemic-shrouded holidays with minimal disruption — for example, by working through a checklist of best practices that include timely testing, scrupulous social distancing and disciplined mask-wearing. Instead, people will need to make serious adjustments as they calculate the risks and rewards of holiday gatherings.

“There’s no easy answer here, just like with everything else. It’s not about safe or unsafe. It’s about figuring out how to balance various risks and keeping risks as low as possible,” Harvard epidemiologist Julia Marcus said.

“This is not a one-size-fits-all issue,” said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases. “It depends on what risk you want to take.”

Fauci then summed up the fundamental conundrum: “It is difficult if not impossible to quantify what that risk is.”

Since the start of the pandemic in the United States, scientific uncertainties and mixed messaging from government leaders have left people in the position of playing amateur epidemiologist, forced to calculate the relative hazards of different activities and the benefits of specific precautions.

People have to estimate their vulnerabilities, and that of friends and relatives, based on age, underlying health conditions, occupational exposures, access to health care and the level of coronavirus transmission in their community and in the places where their holiday guests are coming from.

Donald Milton, a University of Maryland professor of environmental health, said one of his medical colleagues decided to cancel the usual family plans for Thanksgiving: “He’s buying a very small turkey. Or maybe it’s a hen. Or maybe it’s just a chicken breast.”

People have had to make their holiday calculations, even as the scientists and doctors have adjusted their own ideas and guidance. On Wednesday, the Centers for Disease Control and Prevention changed the definition of what “close contact” with an infected person means — it’s now 15 minutes of cumulative exposure within six feet within a 24-hour period, instead of 15 consecutive minutes.

Milton said he and his wife will have a limited family gathering that relies in part on air filters to keep the indoor air as clean as possible. He is an expert on aerosols and has contended the coronavirus can be transmitted at distances much greater than the six-feet standard widely used in the guidance about physical distancing.

“There’s no magic line at six feet. Six feet is a nice handy rule. It doesn’t mean you’re unsafe on one side of the line and safe on the other side,” Milton said.

There remains debate about whether aerosol transmission is a major or minor driver of the pandemic. What’s certain is that, other than completely isolating oneself, there is no way to drive risk to zero.

Even as many experts warn about the risks, there is another factor people must take into account: the psychological benefits of being with family and friends. Fauci said he respects the decisions of people who choose to get together even if some family members have elevated risk factors.

“There are some families that are so frustrated with not seeing each other for so long, they’re going to say, ‘Hey, I’m going to take the risk. My mental health of seeing my children or grandchildren is so important to me that I’m going to take the risk.’ I have no problem with that if that’s what people want to do,” Fauci said. “But they need to realize there is a risk.”

Fauci has revealed that his family will not be getting together for Thanksgiving because, at age 79, he is at elevated risk for a severe outcome from covid-19. His three adult daughters would have had to fly to and from Washington to come home, potentially becoming exposed in transit, and the family agreed the risk wasn’t worth it.

Marcus has been outspoken on the need for public-health officials to offer people practical guidance that goes beyond prohibitions and various forms of scolding. She has criticized the news media, for example, for “beach shaming” — running photographs and videos of people having fun at the beach, even though that is a relatively low-risk environment if people maintain social distancing.

“You can’t just tell people no. It doesn’t work for HIV. It doesn’t work for pregnancy. It doesn’t work for substance use. It doesn’t work for any other area of public health,” Marcus said. “We want to help people find the lowest risk place they can be in their lives that’s healthy and sustainable for them. If you’ve reduced risk to zero, you’ve probably compromised your health in other ways.”

So this is advanced math for everyone, a make-your-own-algorithm challenge. The days are getting darker quickly, and the holidays are looming larger.

The next big hurdle is the quasi-holiday of Halloween, which is not only a favorite of children but a favorite party night for young adults, heavily promoted by commercial interests.

Start with the kids: Is it safe to trick-or-treat?

“That’s potentially a perfect way to spread the virus,” said Emmanuel “Chip” Walter, chief medical officer of the Duke Human Vaccine Institute.

But he said he understands many people will feel compelled to do it anyway, and he suggested they move in small groups and use hand sanitizer for when children reach into a bowl to retrieve candy. He suggested that people could leave candy at the edge of the property in individually wrapped bags — “realizing that little kids might grab them all.”

Without an approved vaccine and with limited therapeutics to combat the coronavirus, the medical community is counting on the public to use common sense.

“Avoid buffets, potlucks or other self-serve options,” warned a news release Wednesday from the Infectious Diseases Society of America.

Many families may hope timely testing will solve the problem of people converging from distant points. But some experts say testing won’t solve all the holiday issues. The most sensitive tests, the PCR genetic tests, do not typically provide a result for several days. The rapid-response antigen tests are usually faster, but they aren’t as sensitive and can miss some infections.

Timing is everything: A person who is exposed in transit, say on an airplane or in an airport, probably would not immediately develop an infection that could be detected by a test, Walter noted.

“Somebody may have been exposed and may not yet have a totally developed infection,” Walter said.

Tests, Walter said, “can be reassuring but potentially falsely reassuring. I wouldn’t put all my stock in testing.”

Leana Wen, the former Baltimore health commissioner and a contributing columnist to The Post, believes a combination of quarantining, testing and precautions during travel could lower the risk for someone traveling home for the holidays from some distant location. The quarantining should last 14 days before a rapid-response test, which ideally would be just before the holiday — for example, the Wednesday right before Thanksgiving.

But the very first thing people should do is talk through the issue and decide on a policy of risk tolerance, she said.

“This is the year to say, ‘No, thank you, we cannot participate because we cannot subject ourselves to this level of risk,’ ” Wen said. “Everyone needs to define their limits, and that needs to be respected. And that may include families not being able to see one another this winter.”

The CDC on Monday issued official guidance on holiday celebrations, reminding people that the safest thing to do is stay home, sticking with people already in the household. If you get together with other people, limit the numbers and the duration of the event. Outdoors is better. Masks are essential, as is six feet of physical distancing and good hand hygiene.

But there are limits to risk reduction. A person can’t eat Thanksgiving dinner, drink holiday eggnog or ring in the New Year with champagne while wearing a mask.

The question of who gets to participate is also tricky. The CDC says people with “increased risk” from the coronavirus should not attend in-person holiday gatherings. The CDC defines that as “older adults” and those with chronic medical conditions.

Who are “older adults?” The CDC simply states — accurately — that risk increases with age, noting people in their 50s have a greater risk than people in their 40s, and people in their 60s and 70s a greater risk than people in their 50s. People older than 85 are at greatest risk.

There are more than 100 million people in America over age 50. There are millions more who are younger but have chronic conditions such as obesity and diabetes. Taken literally, the CDC guidance implies that a very large percentage of the American population should not join friends and family at the holiday table.

This is a confounding virus: It is not predictable in its effects. Even very elderly people usually recover from the coronavirus. Although young people are much less likely to have a severe illness, it’s not impossible.

A severe outcome from covid-19 is thus, for most people, a low probability but high consequence event — the threat of which can’t be perfectly estimated.

Fauci said he isn’t trying to ruin everyone’s holiday season.

“I don’t want to come across as the Grinch that stole Thanksgiving,” Fauci said, but added, “It is something that people need to be concerned about.”

Alexander urges everyone to think long-term — and imagine a better holiday season next year.

“We really need the public thinking about the larger good. In this case, it’s the health of the nation,” she said. “This holiday season is going to be different. It just is. It’s not what I want — but this is the pandemic we’ve been handed, so we’re going to have to celebrate differently.”