Thursday’s White House news conference began and, within minutes, so did the cries of alarm on Twitter:

“Where is Dr. Fauci?”

“What happened to Dr. Fauci?

“Dr. Fauci, paging Dr. Fauci. Where the heck is Dr. Fauci?????”

Seventy-nine-year-old Anthony S. Fauci has become the grandfatherly captain of the corona­virus crisis. Through unrelenting appearances both in the media and onstage with the president and his lieutenants, the longtime head of the National Institute of Allergy and Infectious Diseases has been a reliable constant in a time of uncertainty.

But now, for the second day in a row he was absent. And it was making people nervous.

Had he been sidelined? Was he (extremely hard swallow) . . . sick?

Turns out Fauci was back at the office, pushing ahead with work on a potential vaccine, having briefed the president before the presser.

But the worry was telling.

As recently as a few weeks ago, it might have seemed as if the gravest threat facing the country was the fact that reality had split along partisan lines, creating unresolvable disagreements about what was happening in America and why.

Now a public-health catastrophe has remade our reality and pushed Fauci into the spotlight as a figure that might have seemed impossible less than a month ago: a government expert with an unwelcome message who is nonetheless regarded as a truth-teller, if not a godsend, by the president, Democratic leaders and media figures alike. Surviving may require a single set of facts; and Fauci — a slight, bespectacled man with a Brooklyn accent and sympathetic eyebrows — has them.

But are facts enough to sway a president who often trusts his own feelings more than other people’s expertise and who tends to lash out at those who contradict him? When Fauci returned to the briefing room for Friday’s news conference he was there to see Trump tout the potential benefits of a malaria drug that is not yet proved to be an effective treatment for covid-19 and blow up at a reporter who asked a question he considered “nasty.” Despite his admonitions against face-touching, Fauci rubbed his forehead.

It fell to the doctor to lower the temperature in the room, delicately bridging the gap between Trump’s feelings and his own scientific approach.

“It’s the hope that it will work versus proving that it will work,” Fauci said. “So, I don’t see big differences here.” He yielded the lectern as Trump chimed in: “I agree.”

Kellyanne Conway, a senior counselor to the president, says her boss is a fan of the even-tempered scientist. “The president has been very impressed by what he’s heard from Dr. Fauci. He does like him personally. He respects him professionally.”

“It’s a shame,” says House Majority Leader Rep. Steny H. Hoyer, a Maryland Democrat, “that at the first hint of this we didn’t just say to Tony Fauci, ‘You’re in charge, you have all the power you need, tell us what needs to be done.’ ”

After all, we weren’t ready for this, and he was. He’s been preparing for decades.

If ignorance is bliss, it would stand to reason that Fauci would be miserable.

He seems restless, at least. For years he ran seven miles a day. Lately it’s been more like three-and-a-half miles, most of them power-walked, and about five hours of sleep a night. People have always asked the scientist — who’s been at the forefront of battles against AIDS, West Nile virus and anthrax — one question: “What keeps you up at night?”

His answer, he says, was always the same:

“A respiratory-borne illness that’s easily spread from person to person that has both a high degree of morbidity and mortality,” he said in a phone interview from his office at the National Institutes of Health. “And unfortunately for us that’s exactly what we’re dealing with right now.”

The doctor’s mandate now is not just to help the White House brace for impact but also to convince all of America to buy into a terrifying prognosis, along with prescriptions — washing hands relentlessly, maintaining distance from friends and loved ones — for how to minimize the pain the virus could inflict on society. That includes the legions of Trump supporters who continue to joke about a “beer virus” — Corona, har har — and insist that the whole thing is being intentionally blown out of proportion to damage the president (who called the alarm over the coronavirus threat a Democratic “hoax” in late February).

It’s a daunting task, but Fauci has a few things going for him: not just his expertise but his bedside manner.

The grandson of Italian immigrants, Fauci was born in New York the year before the United States entered World War II and grew up in an apartment above his father’s pharmacy. He delivered prescriptions to customers and decided to pursue medicine early on, eventually graduating first in his class at Cornell Medical College. As a clinical doctor, caring for patients, he cultivated perfectionism. “I came to the conclusion that I owed it to these people, who were really quite ill, to give it everything I possibly could,” he says. “I tried to be as perfect as I could. Even though I know I’m not perfect.”

As a researcher, he made waves in scientific circles for research on immune regulation that led to breakthrough advances in the treatment of rheumatologic diseases. But he never lost his gift for retail medicine, which has matured into a straight-talking-uncle-from-Brooklyn charisma.

“One of the reasons everybody loves this guy is that he combines this extraordinary intellect with a demeanor that does not confront you with, ‘I’m the smartest guy in the world,’ ” says Hoyer, the Maryland Democrat, who has worked closely with Fauci for decades. “It’s not fancy words or fancy concepts, no attempt to awe you, but to communicate what is serious and how we ought to respond.”

In 1984, as the AIDS epidemic raged, Fauci found himself at the center of his first political storm. He had made the disease his urgent focus, both as an administrator and scientist; two of his publications on how HIV affects the body would be among the most frequently cited papers in AIDS research. But the scientific literature was only one front in that war. People were dying, and activists were furious with the government's response. (Some in the scientific community were adamantly opposed to giving AIDS patients a say in how treatments were pursued. And President Ronald Reagan hadn't even said the name of the disease, even though it had already killed thousands of Americans.)

Fauci, then the newly installed director of NIAID, the institute within the National Institutes of Health that studies allergies and infectious disease, faced pressure from all sides. AIDS rights activists staged elaborate protests demanding changes to regulatory processes that were slowing development of new treatments and more funding to support the search for a cure. Fauci was called a murderer in op-eds, and protesters burned him in effigy during protests outside NIH headquarters.

While other government officials refused to even meet with patient advocates, Fauci not only welcomed them to his office, he invited them to dinner parties.

Fauci asked his deputy, a gay man with a well-appointed townhouse on Capitol Hill, to host the wine-fueled evenings. As the activists drove down from New York, they would remind one another to be firm and focused with their demands and to be careful not to fall fully into the Tony Fauci charm vortex, according to Peter Staley, an activist with a New York-based group named ACT UP.

They always brought along Mark Harrington, an ACT UP member who would later win a MacArthur “genius grant,” because he could press Fauci on the science. “Otherwise you’re kind of in awe of the guy and you kind of become deferential,” Staley says.

The activists were aware that the dinner parties were as strategic as they were friendly, he says, and afterward they would try to sort out when Fauci had been handling them and what details he’d been carefully hedging on. “We knew he was playing a game of ingratiating, which he has done with every president that he has worked under. He’s incredibly skillful at it.”

Fauci had to manage not just the needs of activists and science colleagues but also those of his political superiors. When it came to dealing with Reagan — and each of the five presidents he’s served since — the doctor has heeded the advice of a friend who spent several years in the Nixon administration:

“When you go to the White House, always say, in the back of your mind, that this may be the last time I’m going there because I might have to tell this president something he doesn’t like.”

Speaking truth to power is recommended, but individual outcomes may vary.

“Depending upon the character of the president, if you give bad news they may say, ‘I don’t want this guy around anymore, he’s causing trouble,’ ” Fauci says. “So the first thing I decided was I would only speak the truth, based on the evidence I had and my purely clinical scientific judgment.”

Fauci knew that commitment would be tested with Trump and whatever public-health threat emerged under his watch — just as it had been with George H.W. Bush and AIDS, Bill Clinton and West Nile virus, George W. Bush and anthrax and Barack Obama and Ebola. After Trump was elected, Fauci wrote papers describing his work on previous epidemics. He says the series ended with a picture of Trump and a question: “What’s next?”

“Certainly,” he remembers writing, “this president and this administration will be challenged with an outbreak of an infectious disease, just like every previous president that I’ve been involved with.”

“And sure enough, to my dismay, it’s happened,” Fauci says now. “It’s happened.”

The doctor has been stockpiling capital in Washington for years. His political superpower, say those who’ve worked with him, is his ability to convert whoever happens to be in front of him — a patient, a medical student, a U.S. president — into an acolyte.

“Fauci is sort of like the Rolling Stones, a name that everyone mentions as the leader in the field,” says Matt Schlapp, chairman of the American Conservative Union and unofficial adviser to the president.

Schlapp recalls a moment in a presidential debate between George H.W. Bush and Michael Dukakis when Bush was asked to name one of his modern-day heroes, and pointed to Fauci. “He had this sterling reputation with both Republicans and Democrats. He seems to be the man for all seasons in these types of situations.”

But storied credibility around Washington doesn’t necessarily guarantee survival like it used to. Trump came to the White House with a deep skepticism of experts and government officials who weren’t willing to change their way of doing things to better serve his political interests.

Reining in the potential devastation of the coronavirus pandemic would rely heavily on buy-in from the public, which would rely heavily on buy-in from the president. Fauci had directed his “SWAT team” of vaccine researchers to start working on a vaccine back in early January, after Chinese officials published the sequence of the virus. But as it spread throughout Asia and Europe, Fauci and his colleagues at NIH and the Centers for Disease Control and Prevention faced a different challenge: preparing the nation for a potentially cataclysmic outbreak while the president repeatedly played down the threat.

“We have it totally under control,” the president said during a Jan. 22 interview with CNBC. At a news conference a month later he projected that the number of Americans with the coronavirus would, within a couple days, be “going to be down to pretty much close to zero.” (Neither assessment was shared by scientists with a deep understanding of infectious diseases.)

Dithering and denial can have grave consequences. Though South Korea detected its first case of coronavirus on the same day as the United States, it began developing a test almost instantly and as a result has seen a sharp decline in new cases of the virus. You can’t stop a pandemic unless you know where it is.

Fauci says he and the other leading public health officials on the White House Task Force that was assembled to address the crisis took a unified approach in persuading Trump of the situation’s gravity: “Well, you know,” Fauci says. “If at first you don’t succeed, try, try again. It’s just continually giving him the facts.”

Emergency interventions were sometimes necessary. As the number of U.S. cases of covid-19, the illness caused by the coronavirus, began to increase in the United States, Fauci did two things that had been taken as grievous offenses when committed by other government officials serving under Trump: He has publicly corrected him on more than one occasion; and he has taken up a lot of the spotlight, appearing on so many shows that Trump referred to Fauci in a recent news conference as a “television star.”

When Fauci interrupted Trump during a televised meeting a few weeks back, Staley the AIDS activist and now a friend of Fauci’s — says his Facebook feed lit up with people worried that Fauci would be fired.

“I was like, ‘No, you don’t get it. He’s like the only person in Washington who Trump can’t fire,’ ” Staley recalls. Fauci is politically invincible, he says, because of “the wall of bipartisan support around him. It’s like a moat.”

Or an immunity.

He's not immune, of course. Not to illness. Not to failure.

Fauci is nearly 80 years old, putting him in a danger zone for potential victims of covid-19 — a fact not lost on the many anxious news-watchers who have cringed to see him arrayed with administration officials in a non-social-distancing way during news briefings. He may have overtaken Justice Ruth Bader Ginsburg as the most digitally doted-on senior citizen in our government.

He’s heard that people across the country are praying for him, willing him to stay healthy.

“Well,” he says in response, “that’s very nice.”

But Fauci knows that time is running out, and not just for him. Unless the virus’s advance can be blunted in the coming weeks, the results could be catastrophic. He’s not perfect, and neither are we. From this point on, Fauci says, “it’s going to be a race.”

And so, he seems to transcend time and space, appearing in all media at all times. On Wednesday he turned up on “Pardon My Take,” a sports podcast, albeit about 45 minutes into an episode that led with news of Tom Brady’s departure from the New England Patriots. On Thursday he chatted with Mark Zuckerberg on Facebook Live. He’s been a fixture on cable news. “I don’t know where you’re getting the energy from, doctor,” CNN host Chris Cuomo told him during an appearance on Wednesday.

The truth is, he’s been tired. The coronavirus threat is “driving me nuts,” he told The Washington Post on Tuesday night. He spent decades preparing for this moment, yes, but the thought of its arrival was also the thing that kept him up at night. And now that it’s come, Fauci is struggling to get even four hours of sleep a night.

Most people don’t see their worst dream come true, or have to keep themselves together under a national spotlight as the nightmare unfolds.

“What I do is accept it,” he says. “I don’t live for bad things to happen so that people can suffer. I live to respond and prevent suffering and death from something I know inevitably will happen.”

Correction: An earlier version of this story stated that George H.W. Bush praised Anthony Fauci in a debate with Bill Clinton. The debate was with Michael Dukakis.