As one of his first acts as president, Joe Biden Wednesday halted the U.S. departure from the World Health Organization and joined a multilateral vaccine distribution initiative, cementing U.S. involvement in the Geneva-based United Nations agency nine months after former president Donald Trump announced intentions to leave.

The move signaled a return to U.S. collaborative involvement in global health after Trump in April snubbed the organization, claiming it was effectively controlled by China and failed to challenge Chinese health reports during the early stages of the coronavirus outbreak.

The United States’ return was hailed by WHO Director General Tedros Adhanom Ghebreyesus, who tweeted, “We are all glad that the United States of America is staying in the family.”

On Thursday Anthony S. Fauci, the top U.S. infectious-disease expert and chief medical adviser to Biden attended a WHO executive board meeting, and praised the organization’s handling of the pandemic while also indicating the United States would join efforts to scrutinize its relationship with China.

Here’s what you need to know about the WHO and what it does.

What is the World Health Organization?

The United Nations body was founded in 1948 with a mission to promote health around the world. The organization took on a swath of responsibilities, including managing the response to major global health priorities such as tuberculosis and malaria, as well as helping with access to health care around the world.

The organization traditionally has acted as a coordinating authority on health responses and initiatives with government health-care ministries and nongovernmental organizations. Ultimately, as a part of the United Nations, its powers are limited, and it operates at the discretion of governments.

The WHO has six regional offices in Africa, the eastern Mediterranean, Europe, the Americas, Southeast Asia and the Western Pacific. The organization also has some 150 field offices around the world.

Who works there?

The 194 U.N. member states select representatives for the organization’s governing body, the World Health Assembly. The WHA sets priorities for the organization and approves its budget and other activities or policies.

Around the world, the organization says, it employs more than 7,000 people, from doctors, scientists and epidemiologists to administrators, policy experts and economists.

What is its role in fighting the coronavirus pandemic?

Country to country, the organization primarily works by helping local health-care systems train physicians and organize containment and treatment strategies.

The organization holds regular briefings on the virus’s spread around the world, and has aided in the distribution of medical supplies worldwide.

International approval of its response to the pandemic is mixed. In Japan and South Korea opinion of the WHO was low, but in Sweden, Spain and the United Kingdom, people thought the agency has done a better job handling the pandemic than their own governments, according to data from the Pew Research Center released in November.

The agency is also a leader in the multilateral initiative, called Covax, to develop and equitably distribute vaccines. As wealthier countries have started to roll out ambitious vaccination campaigns, lower and middle income countries are lagging behind. Experts predict some countries in the developing world won’t get vaccines until as late as 2024. While the Biden administration will join Covax, it remains focused on vaccinating U.S. citizens. The president aims to vaccinate 100 million Americans in his first 100 days in office.

What is its relationship with China?

Trump criticized the WHO for its early handling of the outbreak especially in relation to Beijing. As information about the respiratory illness filtered out of Wuhan, the virus’s first epicenter, Tedros praised the country’s transparency and swift response. (At the time, Trump also heaped early praise on Beijing for its handling of the virus.)

But international observers thought the WHO was too quick to amplify Chinese data without signaling that it could be inaccurate.

“You had the authority, you had the ability to challenge China, to question China as to what they were doing, and you needed to do that for global health,” said David Fidler, an adjunct senior fellow at the Council on Foreign Relations who has written about and worked with the WHO for years, referring to the organization. “You failed to do it.”

This week, an independent panel criticized the organization for failing to be tougher on China and responding too slowly to the early outbreaks.

The agency’s relationship with Beijing has continued to be tumultuous. A WHO-led mission to investigate the origins of the coronavirus chafed against Chinese bureaucracy and restrictions for nearly a year before it was finally let into the country this month.

What else has it done?

In its more than 70-year history, the WHO has tackled some of the world’s most pressing health issues, including emergencies. It helped eliminate smallpox and all but eliminate polio. It supports the administration of measles vaccines worldwide. More recently it played a role in the 2003 SARS outbreak, the 2009 H1N1 influenza outbreak and the 2014 Ebola outbreak in West Africa.

But it also has had its share of difficulties.

In the 1960s, the WHO “suffered a colossal setback with the failure of global efforts to eradicate malaria,” wrote The Washington Post’s Alan Sipress in “The Fatal Strain: On the Trail of Avian Flu and the Coming Pandemic.” “It was emblematic of a broader resurgence of infectious-disease as microbes mutated, outsmarting new medicines and vaccines, exploiting environmental degradation, poverty, population growth, and humanity’s lapses in vigilance.”

The organization also came under criticism for its response to the 2014 Ebola outbreak, with some health experts saying the body was too slow to act in the face of an outbreak that resulted in more than 10,000 deaths.

During the ongoing coronavirus pandemic, the organization has faced criticism for hewing too close to China and praising the country’s initial response to the crisis.

Who funds it?

The WHO receives funding from its member states and from donors. Each member pays annual dues based on the country’s population and income. Members can volunteer more funds, and these voluntary funds make up the bulk of the WHO’s budget.

For 2020-2021, the WHO has a budget of more than $4.8 billion. The United States is the largest donor in this budget, having pledged more than $400 million a year, much of which is voluntary spending.

A State Department spokesperson told The Post that the United States has committed $893 million to the WHO during its two-year funding period.