A patient of Luana Marques, a clinical psychologist at Harvard Medical School, recently told her, “I am certain that I’m going to contract this virus, and I’m going to die, and my children will be alone.”

Marques walked her patient through the unlikeliness of such a dire outcome — all the improbabilities stacked upon one another. The patient showed no sign of infection with the coronavirus. Most cases of covid-19 are mild to moderate, and people usually recover at home. In every age group, the vast majority of people who contract the virus survive.

“When you’re anxious, the brain has some thinking errors. One of those errors is probability overestimation,” Marques told me.

In this scary spring of the coronavirus pandemic, as shutdowns end and businesses reopen, we’re all engaged in risk analysis, trying to understand our vulnerabilities and figure out what’s safe and what’s not. We have to do this not simply for activities that are inherently hazardous in non-pandemic times, such as driving a car, but for prosaic things like talking to a passing neighbor, opening the mail or rubbing our weary eyes after too many hours staring at the laptop.

We develop personal algorithms with limited information. We have to protect ourselves against an invisible, odorless, submicroscopic threat. And as we try to gauge the dangers, we’re pretty much on our own. The government has been a wildly inconsistent, if not wholly incompetent, counselor in these difficult moments. President Trump has both led the national response and encouraged insurrection against it, while extruding misinformation volcanically. Public health officials on the White House coronavirus task force are routinely forced to do misinformation control. The Centers for Disease Control and Prevention is a treasured institution filled with expertise, but it hasn’t held a news briefing since early March. Governors go their own way, as if entitled to their own epidemiological truths.

All of that heightens our instinct to go it alone and improvise our rules, based on something we found on the Internet or heard from a friend shouting from the sidewalk across the street. Most of us aren’t epidemiologists, and we have no innate grasp of how to think about a new virus. And the type of dispassionate, evidence-based calculation that we’re all trying to make on the fly is something our brains are particularly bad at.

Even science is having a rough pandemic. In normal times, scientific research goes through a vigorous peer-review process prior to publication in a journal. During this crisis, the urgency of the moment has sped things up, and preliminary research often gets national attention and media coverage before outside scientists have had a chance to challenge the findings. At The Washington Post, we cover these preliminary studies with caution, bordering on trepidation, as it is easy to get burned by research in the “interesting if true” category.

Scientists are still trying to understand the exact transmission modes of the virus, the disease progression, the quirky symptoms (blood clots, multiple organ failure, the “second-week crash,” the “cytokine storm”). It’s clearly much worse than the flu; even with unprecedented social distancing that started in mid-March, the United States tallied more than 58,000 covid-19 deaths in a single month.

Weirdly, a large percentage of people experience no symptoms after being infected, as if the virus wafted through them like neutrinos from deep space. This makes risk analysis all the trickier: We can’t really trust anyone to be noninfectious. We can’t even trust ourselves. Simply feeling healthy is no guarantee that you’re not a vector. When is it okay to visit Grandma without putting her in jeopardy?

As a reporter covering this story, I get questions like these from family and friends, and I wish I had rock-solid answers. I sense that some people are letting prudence drift into irrational fear — that they’re suffering from the uncertainties. Is there a way to stay relatively calm amid this nightmare?

Early on, Joshua Gordon, the director of the National Institute of Mental Health and a neuroscientist who has studied how people assess risk, told me that what people need in a time like this is a plan — a pandemic strategy. That seems obvious, but I shared the advice with friends and family: Make a plan. Be in charge. Feel in control.

Some attempts at gathering information heighten our anxiety needlessly. For example, you may have read that a researcher at MIT did an experiment that showed that a cloud of respiratory particles could travel 27 feet. But that doesn’t mean this was sufficient for the virus to be infectious. The CDC at one point noted that traces of the coronavirus had been detected on surfaces on the Diamond Princess cruise ship up to 17 days after the boat was evacuated. But hold on: Isolated genetic fragments of the virus aren’t viable. You can’t get infected from being in a room that a sick person stayed in weeks ago. Nor is it likely that you will get the virus from a person briefly passing by, or coughing at a distance greater than an NBA three-pointer. This disease typically spreads among people in close, prolonged contact in confined spaces. Nursing homes, prisons, cruise ships, households — that’s where it has clustered.

Unfortunately, it’s hard for most of us to stay evidence-based as we develop pandemic strategies. “Our cognitive systems are not like calculators. They’re not adding up pros and cons. They’re making guesses based on all your past experiences,” Gordon said. “We don’t do the math. . . . We operate more like a handwriting recognition machine that’s trying to make its best guess of what’s written down on a piece of paper.”

Excessive or irrational fear can be dangerous to your health — and misjudging the risk from the virus can lead to decisions that make things worse. In Cleveland, cardiologist Steven Nissen says many people with heart conditions are so fearful of the virus that they’re not showing up as they usually would at the Cleveland Clinic. “I don’t want people sitting at home with a heart attack,” Nissen said. “The enemy here of common sense is inordinate fear. . . . We wouldn’t tell people to starve themselves to death because they’re afraid to go to the grocery store.”

How we process risks and fears can be situational, pivoting on our proximity to the gravest threats and perhaps the status of our jobs or the local economy. A front-line health-care worker, a nursing home attendant or the caretaker of an elderly parent is likely to have a different view than a cooped-up teenager or a laid-off bartender. Risk analysis also changes with experience: After weeks of quarantine, a person still uninfected may wonder if this was all a big overreaction. Success looks like overkill.

The epidemiologists warned about this: Their models suggest that the best practice is to drive the infection rate very low before reopening the economy. But the science collides with human nature. Quarantine fatigue is real. Most of us want this whole thing to be over and for the pervasive fear to subside — for hugging a friend to be safe again, and grocery shopping to be calming rather than unnerving.

Vincent Munster, a virologist at the Rocky Mountain Laboratories, part of the National Institute of Allergy and Infectious Diseases, understands the coronavirus better than almost anyone. He has samples of it in his lab, one of those Level 4 biosafety containment facilities that can study scary germs. Munster was the senior author of a paper published in March that showed that the virus could remain viable in the air for up to three hours and on certain surfaces, such as metal, for up to the three days. That research is one of the major reasons that many people are careful with any packages showing up in the mail or being delivered from the store.

But Munster has always made clear that the virus, when outside a host, degrades quickly, becoming less and less infectious in a matter of hours. He doesn’t think “fomite” transmission — the virus spreading from contaminated surfaces — is nearly as significant as direct, person-to-person transmission (though he lives in Hamilton, Mont., far from most hot spots). “The highest likelihood of being infected is really from being in close contact with an infected person — either too close, like talking to someone, or touching,” he told me recently. “That fomite transmission is likely playing a role, but probably a smaller role than actually being in contact with somebody.”

He takes the usual precautions, including washing his hands often. But he’s not haunted by the virus. He’s not frightened by his groceries, his mail or a package from UPS. Of fomite transmission, he says simply, “I personally am not worried about it at all.”

joel.achenbach@washpost.com