On Monday, Dr. Anthony Fauci announced that he will depart from his positions as President Joe Biden's chief medical adviser and director of the National Institute of Allergy and Infectious Diseases at the end of the year.

The 81-year-old won't be fully retiring just yet. In his announcement, Fauci didn't specify his future plans, but said he'll pursue a new chapter of his career that will continue to advance public health and focus on mentoring the "next generation of scientific leaders."

It's a new step for the longtime public servant, who quickly became the public face of the government's Covid-19 response after the pandemic first shuttered the nation in March 2020. His staunch commitment to science and embrace of mitigation measures like masks and temporary business closures turned him into a political lightning rod who frequently battled misinformation, at times from former President Donald Trump.

But Fauci had a far-reaching impact on the nation even before Covid pushed him into the spotlight.

Fauci spent his 38-year tenure as NIAID director leading pivotal efforts to research new and reemerging diseases and develop tools for protection. He advised seven different U.S. presidents, beginning with Ronald Reagan in the 1980s, on infectious disease threats like HIV/AIDS, the West Nile virus, the anthrax attacks, pandemic influenza, Ebola and Zika. In the lab, his groundbreaking research extends as far back as the late 1960s.

"Thanks to the power of science and investments in research and innovation, the world has been able to fight deadly diseases and help save lives around the globe," Fauci said in a statement on Monday. "I am proud to have been part of this important work and look forward to helping to continue to do so in the future."

Fauci's leading role in fighting the HIV/AIDS crisis

Before Covid, Fauci was widely known for leading the nation through another crisis: HIV/AIDS. "My career and my identity has really been defined by HIV," he told The Guardian in a 2020 interview. 

Fauci played a critical role in HIV/AIDS research in the early 1980s, making groundbreaking contributions that helped scientists understand how HIV destroys the body's natural defense system and progresses to AIDS.

After becoming the NIAID director in 1984, Fauci clashed with LGBTQ+ activists who were frustrated with the agency's slow search for HIV treatments. One prominent activist, Larry Kramer, called Fauci a "murderer" in an open letter in 1988 — pointing to clinical trials Fauci oversaw for HIV treatments involving antivirals like AZT. The antiviral came with significant drawbacks, from low blood cell counts and liver problems to a relative lack of effectiveness on its own.

In 1990, roughly 1,000 HIV activists protested Fauci outside the National Institute of health — carrying banners, mock coffins for the doctor and, at least for one protestor, "Fauci's head on a stick," according to the Washington Post.

Notably, and perhaps surprisingly, Fauci eventually became a respected ally by engaging with activists and "doing more listening than talking," Georges Benjamin, the executive director of the American Public Health Association, who has worked with Fauci since 1990, tells CNBC Make It. 

Fauci ultimately helped change the way the government handled clinical drug trials, increasing the number of patients who had access to experimental HIV treatments. He later worked with President George W. Bush to design a global program to provide treatment to people with HIV, which "may be the most impactful thing I have done in my career," he told Politico last month.

The program, known as PEPFAR, saved an estimated 21 million lives across more than 50 countries. Bush awarded Fauci the Presidential Medal of Freedom in 2008, five years after the program's launch.

Today, HIV/AIDS patients can live for years with the disease, and prevent transmission with daily drug therapies. While there is no HIV vaccine yet, Fauci continues to lead efforts to develop one.

"If he weren't retiring in December, I'd imagine him working to his very last breath until there was a cure for AIDS," Dr. Gregg Gonsalves, an associate professor of epidemiology at the Yale School of Public Health and longtime AIDS activist, wrote Tuesday in The New York Times.

Fauci's groundbreaking scientific lab work

Before Fauci became a public figure, his lab research defined his career.

One of his first scientific breakthroughs came after he joined NIAID as a clinical associate in 1968, according to the Washington Post: Fauci, then in his 30s, discovered how to re-dose cancer drugs in a way that turned the 98% mortality rate of vasculitis — a rare inflammatory disease where your blood cells attack your blood vessels — into a 93% remission rate. 

In 1980, Fauci was appointed chief of the NIAID'S Laboratory of Immunoregulation, a position he still holds today. There, he helped pioneer the field of human immunoregulation, which refers to understanding how to regulate the human immune system's response to bacteria, viruses and other harmful substances. 

As the NIAID director, Fauci has overseen an extensive portfolio of research on preventing, diagnosing and treating infectious diseases from HIV/AIDS and other sexually transmitted diseases to respiratory infections and diarreheal diseases, among others. NIAID also supports research on transplantation and immune-related illnesses, such as asthma and allergies.  

A 2022 analysis of Google Scholar citations ranked Dr. Fauci the 44th most-cited living researcher in the world. Fauci also ranked 9th out of more than three million authors in the field of immunology by total citation count between 1980 to 2022, according to the Web of Science.

"At the end of the day, he wanted to be a scientist. He enjoyed being in the lab and he was good at it," Benjamin says.

Sharp criticism of Fauci from the right

Some Americans have cast the straight-talking Fauci as a pandemic-era hero for helping guide Americans through Covid's peak in 2020, when almost everything about the virus was uncertain. He's also faced pushback from Covid skeptics and opponents of public health measures like social distancing, temporary business closures and masking.

That includes Trump, who often publicly sparred with Fauci on how to approach the pandemic. Fauci, in turn, frequently corrected the former president's false statements about Covid.

Other Republican lawmakers have accused Fauci of playing a role in Covid's origins — an unproven claim, to date — with some vowing to investigate Fauci if Republicans win control of Congress this fall.

Amid the backlash, Fauci has remained publicly committed to science over politics. He's consistently urged people to stop gathering in large groups, isolate if sick, wash their hands and exercise personal responsibility to avoid spreading Covid. The methods have been scientifically found to effectively reduce Covid transmission, according to a systematic review published in May 2021 by BMC Public Health.

"He always spoke to science, committed to what was right," Benjamin says.

Fauci also helped accelerate the development of the first Covid vaccines. He and other NIAID researchers partnered with drugmaker Moderna to create its shot, which has an efficacy rate above 90%, in less than a year.

Today, Fauci continues to laud vaccines and booster shots as an effective protection method, often fighting directly against disinformation fueled by a national anti-vaccine movement that's only grown since the Covid pandemic started: Almost 150 anti-vaccine social media accounts saw a nearly 20% increase in followers between 2019 and 2020, according to a study from the Center for Countering Digital Hate.

"I think that's the measure of the man he is," Benjamin says. "He's tried his very hardest to stay above the political fray. And most of all, he's always tried to speak truth to power.

Sign up now: Get smarter about your money and career with our weekly newsletter

Don't miss:

Dr. Fauci: BA.5-specific booster shots are the country's 'best guess' for dealing with Covid this fall

Omicron-specific Covid vaccines could finally be here this fall—here's what you need to know