President Trump’s decision to withdraw the United States from the World Health Organization could have harsh consequences for countries around the world that rely on the agency to achieve important health goals, including routine immunizations, outbreak control and nutrition programs.

But it could also have unfortunate, unintended repercussions for Americans.

Disengaging from the W.H.O. would rob the United States of crucial information about emerging outbreaks like mpox and resurgent dangers like malaria and measles, public health experts said. It may also give more power to nations like Russia and China in setting a global health agenda, and it could hurt the interests of American pharmaceutical and health technology companies.

The W.H.O.’s work touches American lives in myriad ways. The agency compiles the International Classification of Diseases, the system of diagnostic codes used by doctors and insurance companies. It assigns generic names to medicines that are recognizable worldwide. Its extensive flu surveillance network helps select the seasonal flu vaccine each year.

The agency also closely tracks resistance to antibiotics and other drugs, keeps American travelers apprised of health threats, and studies a wide range of issues such as teen mental health, substance use and aging, which may then inform policies in the United States.

“There’s a reason why there was a W.H.O.,” said Loyce Pace, who served as an assistant secretary of health and human services under former President Joseph R. Biden Jr. “It’s because we saw value, even as a superpower, in the wake of the world war to come together as a global community on global problems.”

“America, no matter how great we are, cannot do this work alone,” she said.

Though it will take a year for the withdrawal to take effect — and it is not entirely clear that it can happen without congressional approval — Mr. Trump’s announcement has already prompted drastic cost-cutting measures at the W.H.O.

In a memo to employees, the director general, Dr. Tedros Adhanom Ghebreyesus, announced a hiring freeze and limited renegotiation of major contracts, adding that more measures would follow. He also said all meetings without prior approval should be fully virtual from now on and “missions to provide technical support to countries should be limited to the most essential.”

Late Sunday night, employees of the U.S. Centers for Disease Control and Prevention were instructed, effective immediately, to stop engaging with the W.H.O. in any way. The employees were later told not to participate in meetings or even email conversations that included W.H.O. staff.

The W.H.O. is often criticized as a lumbering bureaucracy, too conservative in its approach and too slow to action. Mr. Trump cited the organization’s “mishandling of the Covid-19 pandemic” as one of the main reasons the United States is pulling out.

Many public health experts have for decades called for reforms of the agency, noting that it is too timid in calling out its members’ missteps, holds a rigid view of what constitutes medical evidence and has too many areas of focus. The criticisms escalated during the pandemic, when the W.H.O. was months late in acknowledging that the coronavirus was airborne and that the virus could spread in the absence of symptoms.

Yet there is no other organization that can match the W.H.O.’s reach or influence in the world, said Dr. Thomas Frieden, who has worked with the W.H.O. for decades, including as a former C.D.C. director.

“Are there lots of things they could be better at? Of course,” he said. But, he added, “are they indispensable? Yes.”

For all its scope, the W.H.O. has a relatively modest budget, totaling about $6.8 billion for 2024 and 2025. For comparison, the health department of the tiny state of Rhode Island spent just over $6 billion in 2024 alone.

The United States is the W.H.O.’s largest donor, accounting for nearly 15 percent of its planned budget.

In the executive order, Mr. Trump complained that the W.H.O. “continues to demand unfairly onerous payments from the United States, far out of proportion with other countries’ assessed payments,” adding that China contributes nearly 90 percent less.

Both of those assertions are inaccurate.

The obligatory fees are calculated according to each country’s population and income, using a formula approved by member states. For the two-year 2024-25 budget, that amount was $264 million for the United States and $181 million for China, a difference of about 31 percent.

Mr. Trump’s claim that China pays much less may have been based on voluntary contributions, which are usually motivated by specific interests such as polio eradication: The United States has so far provided $442 million in voluntary contributions for 2024-25, while China has given only $2.5 million. Even so, China’s total contribution is about 74 percent less than the United States’, not 90 percent.

Mr. Trump’s decision was “not based on sound, factual ground,” said Helen Clark, a former prime minister of New Zealand and former administrator of the United Nations Development Program.

On Monday, the Trump administration halted the distribution of H.I.V. drugs purchased with U.S. aid. Abruptly ending treatment will jeopardize the health of people living with H.I.V. and lead to more infections and may drive resistance to available medications, health experts warned.

The W.H.O.’s programs monitor drug resistance worldwide to antibiotics and medications for H.I.V., malaria and other diseases.

“These are not invincible drugs, and having that ability to know when resistance occurs and why we need to change strategies can be very important,” said Dr. Meg Doherty, who directs W.H.O. programs on H.I.V. and sexually transmitted infections.

“They are things that people in the United States should be aware of and should be concerned could come to them in the future,” she said.

If the United States loses access to the W.H.O.’s information and data sharing, online reports and informal communications may fill some of the void, but they may be muffled, filtered or marred by misinformation. And the W.H.O. and other countries are not obligated to share information, such as genetic sequences, with the United States, let alone heed its advice, if the country is not a member.

“If we’re not there, we don’t get to have a voice at all,” Dr. Frieden said.

The W.H.O. began in 1948 as a branch of the United Nations focused on global health. Over the decades, it led the eradication of smallpox, nearly vanquished polio and has helped control use of tobacco and trans fats.

Countries that do not have the equivalent of a C.D.C. or a Food and Drug Administration rely on the W.H.O. for public health guidelines, childhood vaccinations and drug approvals, among many other health efforts.

“Ministries of health typically won’t move unless there’s a W.H.O. guideline,” said Dr. Chris Beyrer, director of the Duke Global Health Institute and an adviser to the W.H.O.

That dynamic has implications for American businesses, allowing pharmaceutical and health technology companies to operate in countries that adhere closely to W.H.O. recommendations, said Anil Soni, chief executive of the W.H.O. Foundation, an independent entity that facilitates partnerships and funding for the organization.

“The U.S. won’t be at the table to set the evidence and quality standards that enable competitive positioning of U.S. companies and directly lead to U.S. business,” Mr. Soni said.

Mr. Trump and others have criticized the W.H.O. for not holding China accountable early in the pandemic, and for taking too long to declare the Covid-19 pandemic a public health emergency.

But the W.H.O. cannot reprimand its member countries, noted Ms. Clark, who was a co-chair of the Independent Panel for Pandemic Preparedness and Response, which led an inquiry into the W.H.O.’s response to the Covid-19 pandemic.

“W.H.O. has no power to compel countries to do anything,” Ms. Clark said. “It has only the power of persuasion. China was not transparent, and that hindered W.H.O.’s response.”

Mr. Trump has also said that China has too much influence over the W.H.O. But “actually, the real problem is that tiny Pacific islands with 100,000 people have too much power,” Dr. Frieden said.

“W.H.O. works by consensus, and so any country can throw a monkey wrench in and stop proceedings,” he said.

It is unclear whether Mr. Trump can unilaterally sever ties with the W.H.O. Unlike most international agreements, which may stem from executive action or require Senate ratification, membership in the W.H.O. was enshrined by a congressional joint resolution and may have to be dissolved in the same way.

“There’s a very good argument to be made that the president cannot do this himself — that is, without congressional participation,” said David Wirth, a former State Department official and an expert in foreign relations law at Boston College.

If Congress approves, the United States must still give one-year notice of withdrawal and fulfill its financial obligations for the year.

Some experts worry that Mr. Trump’s action will prompt nations like Hungary and Argentina, whose leaders are ideologically similar, to follow suit. Already, Italy’s deputy prime minister has proposed a law to leave the W.H.O.

U.S. withdrawal may also empower authoritarian member states in the organization, like Russia and China. Public health decisions in Russia and China are “much more politically controlled, and that’s a danger to everybody,” Dr. Beyrer said. “None of us wants to live in a world where Russia has a larger voice in global health governance.”

In his executive order, Mr. Trump said the United States would cease negotiations on amendments to the International Health Regulations, legally binding rules for countries to report emerging outbreaks to the W.H.O. But the latest amendments were adopted by the World Health Assembly last year and are expected to come into force in September.

Ironically, it was the first Trump administration that proposed the amendments because of frustration with the lack of transparency from certain countries during Covid-19, said Ms. Pace, who oversaw negotiations during the Biden administration.

Ms. Pace also led negotiations for a pandemic treaty that would allow countries to work together during an international crisis. The treaty had been stalled and may now collapse.