Washington (CNN) Dr. Anthony Fauci has become a rare source of frank honesty from within the White House coronavirus task force in recent weeks , holding firm with an at times overly optimistic President and gently recommending steps forward in the face of crisis.

The director of the National Institute of Allergy and Infectious Diseases -- and a doctor for the National Institutes of Health for more than 50 years -- has even won public praise from President Donald Trump.

Trump went so far as to say Fauci was doing a "tremendous job" and "working long, long hours" during a Rose Garden news conference Friday, just a day after the doctor admitted to members of Congress that the current set of protocols for physicians to request coronavirus tests for their patients "is a failing."

That's a clear contradiction with the President, who has insisted that "testing has been going very smooth."

The veteran doctor also hasn't been afraid to refute Trump to his face . The dynamic was on full display when Trump was asked last week about a timeline for a novel coronavirus vaccine during a White House meeting opened up to reporters.

"I don't know what the time will be. I've heard very quick numbers, that of months. And I've heard pretty much a year would be an outside number. So, I think that's not a bad range. But if you're talking about three to four months in a couple of cases, a year in other cases," Trump said.

But Fauci, who was in the room, immediately corrected the President: "Let me make sure you get the ... information. A vaccine that you make and start testing in a year is not a vaccine that's deployable."

As Fauci explained the timeline, Trump folded his arms.

"So, he's asking the question -- when is it going to be deployable? And that is going to be, at the earliest, a year to a year and a half, no matter how fast you go," Fauci said.

High-profile career

Fauci is no stranger to leading the federal response to national health emergencies.

He joined the National Institutes of Health in 1968 and has been leading the National Institute of Allergy and Infectious Diseases since 1984. Since becoming director, he's advised every American president since Ronald Reagan. In 2008, he was awarded the Medal of Freedom.

His time at NIH in the 1980s was heavily focused on the HIV/AIDS crisis.

"I went from happy years of making people better to a period of close to a decade of the dark years in which most of my patients died," Fauci told the Infectious Diseases Society of America in 2011.

With his promotion to NIAID director, he became a public face of the federal response to the AIDS epidemic and, according to an interview with Vanderbilt University, labeled as an "incompetent idiot" and a "murderer" by AIDS activists.

He said when he brought protesters and activists into the fold, encouraging a dialogue with the NIH, their recommendations made "perfect sense."

Fauci has also worked on the federal response to Ebola, the Zika virus and anthrax scares.

In 2001, amid the anthrax scare following 9/11, Fauci's direct communication style was on apparent. As a debate across Washington was taking place about dangerous the anthrax found in Democratic Sen. Tom Daschle's office was, he said, according to The New York Times : "If it walks like a duck and quacks like a duck, then it is a duck.''

"You can call it whatever you want to call it with regard to grade and size or weaponized or not weaponized. The fact is, it is acting like a highly efficient bioterrorist agent," Fauci said.

Easy communicator

Fauci seems keenly aware of the role he plays for Trump -- and the overall importance of truth during a public health crisis.

"You should never destroy your own credibility. And you don't want to go to war with a president ... But you got to walk the fine balance of making sure you continue to tell the truth," he told Politico last week

Trump rarely tolerates officials who admit failure, as Fauci did this week when asked about coronavirus testing on the Hill.

But so far, the Trump administration doesn't appear to be concerned with Fauci countering more optimistic outlooks from President and the task force, which is largely filled with political appointees.

Two task force sources told CNN's Jim Acosta that the White House is not mad at Fauci for being candid, adding that so far, Fauci's honesty has been appreciated.

"It's helpful," one White House official said.

"So far, so good," another task force official said.

Fauci hasn't been entirely critical of Trump's handling of the coronavirus. Most recently, he said the President's decision to issue travel restrictions on Europe saved lives.

Key role

In recent days, some lawmakers have urged the President to let Fauci be the public face of the administration's response to coronavirus. The President has praised Fauci and pointed out the doctor's place on the task force -- but some of the lawmakers hope Trump will go further and just let Fauci do the talking.

The National Institute of Allergy and Infectious Diseases director has been seen so frequently around Washington lately -- speaking to the media, attending daily White House meetings and delivering testimony on Capitol Hill -- that his voice at times has been audibly hoarse.

Earlier this month at a House subcommittee hearing to discuss National Institutes of Health funding, Connecticut Democratic Rep. Rosa DeLauro suggested lemon, honey and "maybe a shot of bourbon" to ease his voice.

"I'm surprised you have a voice left at all. You appear to have been everywhere. You must have like twins or something," Maryland Republican Rep. Andy Harris joked.

But even during calmer times, Fauci seems to work around the clock.

Last October, before the coronavirus took hold in the US, Fauci was working on a plan for a potential flu shot that doesn't have to be taken every year. He shared with CNN that he works six to seven days a week, from early morning until late at night, even when there isn't a crisis.