Asanda Zondi received a startling phone call last Thursday, with orders to make her way to a health clinic in Vulindlela, South Africa, where she was participating in a research study that was testing a new device to prevent pregnancy and H.IV. infection.

The trial was shutting down, a nurse told her. The device, a silicone ring inserted into her vagina, needed to be removed right away.

When Ms. Zondi, 22, arrived at the clinic, she learned why: The U.S. Agency for International Development, which funded the study, had withdrawn financial support and had issued a stop-work order to all organizations around the globe that receive its money. The abrupt move followed an executive order by President Trump freezing all foreign aid for at least 90 days. Since then, the Trump administration has taken steps to dismantle the agency entirely.

Ms. Zondi’s trial is one of dozens that have been abruptly frozen, leaving people around the world with experimental drugs and medical products in their bodies, cut off from the researchers who were monitoring them, and generating waves of suspicion and fear.

The State Department, which now oversees U.S.A.I.D., replied to a request for comment by directing a reporter to USAID.gov, which no longer contains any information except that all permanent employees have been placed on administrative leave. Secretary of State Marco Rubio has said that the agency is wasteful and advances a liberal agenda that is counter to President Trump’s foreign policy.

In interviews, scientists — who are forbidden by the terms of the stop-work order to speak with the news media — described agonizing choices: violate the stop-work orders and continue to care for trial volunteers, or leave them alone to face potential side effects and harm.

The United States is signatory to the Declaration of Helsinki that lays out ethical principles under which medical research must be conducted, requiring that researchers care for participants throughout a trial, and report the results of their findings to the communities where trials were conducted.

Ms. Zondi said she was baffled and frightened. She talked with other women who had volunteered for the study. “Some people are afraid because we don’t know exactly what was the reason,” she said. “We don’t really know the real reason of pausing the study.”

The stop-work order was so immediate and sweeping that the research staff would be violating it if they helped the women remove the rings. But Dr. Leila Mansoor, a scientist with the Centre for the AIDS Programme of Research in South Africa (known as CAPRISA) and an investigator on the trial, decided she and her team would do so anyway.

“My first thought when I saw this order was, There are rings in people’s bodies and you cannot leave them,” Dr. Mansoor said. “For me ethics and participants come first. There is a line.”

In the communities where her organization works, people have volunteered for more than 25 years to test H.I.V. treatments, prevention products and vaccines, contributing to many of the key breakthroughs in the field and benefiting people worldwide.

That work relied on a carefully constructed web of trust that has now been destroyed, Dr. Mansoor said. Building that trust took years in South Africa where the apartheid regime conducted medical experiments on Black people during the years of white rule. Those fears are echoed in a long history of experimentation by researchers and drug companies in developing countries and in marginalized populations in the United States.

The Times identified more than 30 frozen studies that had volunteers already in the care of researchers, including trials of:

• malaria treatment in children under age 5 in Mozambique

• treatment for cholera in Bangladesh

• a screen-and-treat method for cervical cancer in Malawi

• tuberculosis treatment for children and teenagers in Peru and South Africa

• nutritional support for children in Ethiopia

• early-childhood-development interventions in Cambodia

• ways to support pregnant and breastfeeding women to reduce malnutrition in Jordan

• an mRNA vaccine technology for H.I.V. in South Africa

It is difficult to know the total number of trials shut down, or how many people are affected, because the swift demolition of U.S.A.I.D. in recent days has erased the public record. In addition to the disabled website, the agency no longer has a communications department. And the stop-work order prohibits any implementing agency from speaking publicly about what has happened.

In England, about 100 people have been inoculated with an experimental malaria vaccine in two clinical trials. Now, they no longer have access to the clinical trial staff if that vaccine were to cause an adverse reaction in their bodies. The trial is an effort to find a next-generation vaccine better than the one now used in Africa; that shot protects children against about a third of malaria cases, but researchers hoped to find a vaccine that offered much more protection. Malaria remains a top global killer of children; 600,000 people died of the disease in 2023, the latest figure available.

Had the trial not been frozen, the participants would be coming to a clinic routinely to be monitored for adverse physical effects, and to have blood and cell samples taken to see whether the vaccine was working. The participants are meant to be followed for two years to assess the vaccine’s safety.

A scientist who worked on the trial said she hoped that partners at the University of Oxford, where it was being conducted, were shuffling staff to respond if any participant fell ill. But she was fired last week and no longer has access to any information about the trial. She spoke on condition of anonymity because she feared jeopardizing her ability to work on malaria research the U.S. might conduct in the future.

“It’s unethical to test anything in humans without taking it to the full completion of studies,” she said. “You put them at risk for no good reason.”

Had the stop-work order come later this year, the newly-vaccinated volunteers might have been in an even more precarious position. They were scheduled to be deliberately infected with malaria to see if the experimental vaccine protected them from the disease.

Dr. Sharon Hillier, a professor of reproductive infectious diseases at the University of Pittsburgh, was until this week director of a five-year, $125 million trial funded by U.S.A.I.D. to test the safety and efficacy of six new H.I.V. prevention products. They included bimonthly injections, fast-dissolving vaginal inserts and vaginal rings.

With the study suspended, she and her colleague cannot process biological samples, analyze the data they have already collected, or communicate findings to either participants or the partnering government agencies in countries where the trials were conducted. These are requirements under the Helsinki agreement.

“We have betrayed the trust of ministries of health and the regulatory agencies in the countries where we were working and of the women who agreed to be in our studies, who were told that they would be taken care of,” Dr. Hillier said. “I’ve never seen anything like it in my 40 years of doing international research. It’s unethical, it’s dangerous and it’s reckless.”

Even trials that were not funded in whole or part by U.S.A.I.D. have been thrown into turmoil because they were using medical or development infrastructure that was supported by the agency and is no longer operational. Millions of dollars of U.S. taxpayer funds already spent to start those trials will not be recuperated.

The shutdowns have business consequences as well. Many of those trials were partnerships with U.S. drug companies, testing products they hoped to sell overseas.

“This has made it impossible for pharmaceutical companies to do research in these countries,” Dr. Hillier said.

Another H.I.V. trial, called CATALYST, has thousands of volunteers in five countries testing an injectable drug called long-acting cabotegravir. Participants were receiving bimonthly injections to maintain a sufficient level of the drug in their bodies to prevent H.I.V. infection. Without regular injections, or a carefully-managed end to use of the drug, the participants will not have enough cabotegravir to stop a new infection, but there will be enough in their systems that, if they were to contract the virus, could easily mutate to become drug-resistant, said Dr. Kenneth Ngure, president-elect of the International AIDS Society.

This is a significant threat to the trial volunteers and also to the millions of people living with H.I.V. because cabotegravir is closely related to a drug that is already used worldwide in standard treatment of the virus. Development of resistance could be catastrophic, Dr. Ngure said: “It’s wrong on so many levels — you can’t just stop.”

A clinical trial run by the development organization FHI 360, which implemented many U.S.A.I.D.-funded health programs and studies, was testing a biodegradable hormonal implant to prevent pregnancy. Now there are women in the Dominican Republic with the devices in their bodies without continuing care.

Another trial, in Uganda, was testing a new regimen of tuberculosis treatment for children. The stop-work order cuts those children off from potentially lifesaving medication.

“You can’t walk away from them, you just can’t,” a researcher in that trial said.

The English language is full of wonderful words, from “anemone” and “aurora” to “zenith” and “zodiac.”

But these are special occasion words, sprinkled sparingly into writing and conversation. The words in heaviest rotation are short and mundane. And they follow a remarkable statistical rule, which is universal across human languages: The most common word, which in English is “the,” is used about twice as frequently as the second most common word (“of,” in English), three times as frequently as the third most common word (“and”), continuing in that pattern.

Now, an international, interdisciplinary team of scientists has found that the intricate songs of humpback whales, which can spread rapidly from one population to another, follow the same rule, which is known as Zipf’s law.

The scientists are careful to note that whale song is not equivalent to human language. But the findings, they argue, suggest that forms of vocal communication that are complex and culturally transmitted may have shared structural properties.

“We expect them to evolve to be easy to learn,” said Simon Kirby, an expert on language evolution at the University of Edinburgh and an author of the new study. The results were published on Thursday in the journal Science.

“We think of language as this culturally evolving system that has to essentially be passed on by its hosts, which are humans,” Dr. Kirby added. “What’s so gratifying for me is to see that same logic seems to also potentially apply to whale song.”

Zipf’s law, which was named for the linguist George Kingsley Zipf, holds that in any given language the frequency of a word is inversely proportional to its rank.

There is still considerable debate over why this pattern exists and how meaningful it is. But some research suggests that this kind of skewed word distribution can make language easier to learn.

If these word distributions evolved because they helped learning, scientists might also expect to find similar patterns in other complex, culturally transmitted communication systems. “And whale song is a great place to look,” said Inbal Arnon, an expert on language acquisition at Hebrew University and an author of the new study.

Male humpback whales sing long, elaborate songs, which are composed of a variety of sounds strung together in repeated phrases and themes. All the male whales in a particular humpback population sing the same song, but that song evolves over time — sometimes gradually and sometimes all at once.

“We have song revolutions, and that’s when a song is introduced from a neighboring population,” said Ellen Garland, an expert on humpback whale songs at the University of St. Andrews and an author of the new paper. “So the song type turns up, and then it completely takes over.”

Precisely how that happens remains a mystery, and whale song researchers face a challenge that human language researchers don’t: They’re not native speakers.

So the scientists’ first challenge was to divide the songs into meaningful units, determining where one “word” ended and another began. To do so, they used a quantitative approach inspired by human babies. Infants, research suggests, use basic statistical reasoning to identify discrete words in a continuous stream of human speech; syllables that occur together are likely to be part of the same word.

The researchers transformed humpback whale songs, recorded over eight years in the waters around New Caledonia, into long sequences of basic sound elements, including various types of squeaks, grunts, whistles, groans and moans. Then, they identified “subsequences” of sounds that frequently occurred together — such as a short ascending whistle followed by a squeak — and might be roughly analogous to a word.

The frequency with which these subsequences were used followed Zipf’s law, the researchers found. In 2010, for instance, groan-groan-moan was the most common subsequence, appearing about twice as often as the next most common sequence, which was a moan followed by three ascending cries. The most frequently used subsequences were also generally shorter than the rarer ones.

Humans and humpback whales are not closely related, and whale song does not carry the same semantic meaning that human language does, Dr. Garland said. But both communication systems are culturally transmitted, learned from others in the community and passed down over the generations. “So this really points to the crucial role of learning and transmission in the emergence of structure,” she said.

Shane Gero, a marine biologist who was not involved in the research, said that he found the study “elegant” and convincing. The results raise the possibility that linguistic laws derived from studies of human communication may actually be broader biological principles, he said.

“The fact that maybe they generalize if we know enough and we study long enough, then that’s really interesting,” said Dr. Gero, who is a scientist-in-residence at Carleton University. “Anytime we look deeper and listen longer, we find interesting complexity.”

Indeed, the next step is to determine whether the phenomenon extends to other animals with similar communication systems.

“We should find these statistical properties in any culturally transmitted system of sequential signaling,” Dr. Arnon said. “So we have bats to look at, we have songbirds to look at, we have elephants, maybe, to look at.”

The findings dovetail with another paper published this week, which found that the vocalizations produced by 11 species of dolphins and whales follow one of the efficiency rules observed in human language. The rule, known as Menzerath’s law, holds that the longer a sequence becomes, the shorter its individual components tend to be; long sentences, for instance, tend to have shorter words.

The study, which was published in Science Advances on Wednesday, documented this same pattern in a diverse array of cetaceans, including humpbacks, which produce melodic songs; sperm whales, which issue sequences of clicks; and bottlenose dolphins, which are known for their whistles.

“Regardless of what their vocalizations are used for, they all seem to try to communicate as efficiently as they can,” said Mason Youngblood, a postdoctoral researcher at Stony Brook University and the author of the study.

The pattern, which has also been documented in birds and nonhuman primates, may have evolved as a way to reduce the costs of communication.

“Things like bird song and whale song are very hard to learn,” Dr. Youngblood said. “And then when you sing, it’s very energetically costly. It can attract the attention of predators. And so because of that, you would expect communication systems to evolve to cut those costs wherever it’s possible.”

Barely a week ago, Israelis rallied in Tel Aviv’s Hostage Square, waving placards thanking President Trump and his Middle East envoy for their role in helping secure an initial cease-fire deal in Gaza and getting some hostages freed.

Many of them were hoping that Mr. Trump would strong-arm Israel’s long-hesitant prime minister, Benjamin Netanyahu, into agreeing to negotiate an end to the war with Hamas and get the rest of the hostages released when the two leaders met in Washington on Tuesday.

Instead, they woke up to news of Mr. Trump’s fantastical idea of removing the population of roughly two million Gazans from the devastated enclave to make way for a glittering, American-owned Middle Eastern Riviera.

Far-fetched as Mr. Trump’s vision for Gaza may be — the Arab world has roundly rejected it and any forcible removal of a population violates international law — it abruptly shifted attention away from the future of the cease-fire deal, whose initial, six-week phase is due to end in early March.

As Mr. Trump sketched out his grandiose plans for Gaza, he placed little public pressure on Mr. Netanyahu to proceed with talks via Qatari and Egyptian mediators to turn the temporary cease-fire into a permanent cessation of hostilities. That left Israel with a wide berth on how it might deal with Gaza next.

The talks, which were meant to start this week, are now up in the air. And Mr. Netanyahu will leave Washington with Mr. Trump’s endorsement of what far-right members of the Israeli government have effectively been calling for: the mass migration of Palestinians from Gaza.

That leaves the fate of the hostages still held by Hamas in question as the militant group assesses how to move forward and many Palestinians worrying about whether the war might resume again.

“On the one hand, we are very grateful for what Trump has been doing,” said Idit Ohel, whose son, Alon Ohel, 23, was kidnapped from a bomb shelter as he tried to flee a music festival during the Hamas-led assault of Oct. 7, 2023, which started the war.

“Now,” Ms. Ohel said of Mr. Trump, “I don’t understand the implications of what he is saying or how this is going to bring my son home.”

Mr. Netanyahu, in an interview with Fox News late Wednesday, hailed Mr. Trump’s idea as “remarkable,” saying it should be “pursued,” drowning out any talk of the details of how to move cease-fire negotiations forward.

And on Thursday morning, Mr. Netanyahu’s loyal defense minister, Israel Katz, issued a statement saying he had instructed the Israeli military to prepare a plan to facilitate the exit of “any resident of Gaza who is interested to leave to any place in the world that agrees to accept them.”

The initial phase of the cease-fire deal took effect on Jan. 19 and provides for Hamas to release 33 hostages in exchange for hundreds of Palestinians imprisoned in Israel. About 79 hostages remain in Gaza, at least 35 of whom are presumed to be dead.

Talks were due to start on Monday — Day 16 of the deal — on a second phase, which is supposed to result in the rest of the living hostages being released and to usher in a permanent cessation of hostilities. That would mean a full withdrawal of Israeli troops from Gaza.

Wording about the transition to the second phase had been left intentionally vague, since Israel and Hamas are holding out for mutually exclusive demands.

Mr. Netanyahu has vowed to continue the war until Hamas no longer holds sway in Gaza and to resume fighting, if necessary. Hamas refuses to give up control or disarm.

In repeated statements in Washington, Mr. Netanyahu laid out his three priorities for Gaza, with the hostages only coming in second.

“In Gaza, Israel has three goals: Destroy Hamas’s military and governing capabilities, secure the release of all our hostages and ensure that Gaza never again poses a threat to Israel,” he said.

Mr. Netanyahu could stand to lose his own grip on power, with the far-right flank of his governing coalition having threatened to quit if he ends the war in Gaza with Hamas still in control there.

As of Thursday, no Israeli delegation had yet set out to Doha, Qatar, for negotiations, according to two Israeli officials who were not authorized to discuss the sensitive issue publicly.

Mr. Trump also sounded less committal than he has in the past about the fate of the hostages and ending the war, saying it was unclear if the cease-fire would hold. He said he would like to get all the hostages out. “If we don’t, it will just make us somewhat more violent,” he said, possibly indicating U.S. backing for a resumption of the fighting.

In the Middle East, analysts were parsing what Mr. Trump’s tectonic diversion on the future of Gaza might mean in the more immediate term.

“I think what he did was throw the old checkers board off the table and replaced it with Monopoly,” said Kobi Michael, an expert in the Israel-Palestinian conflict at the Institute for National Security Studies at Tel Aviv University. “He didn’t just change the rules of the game but the game itself,” he said.

Both Israel and Hamas are likely to want to buy time — Hamas to rehabilitate itself and its forces after 15 grueling months of war and Mr. Netanyahu to keep his right-wing coalition together — and may try to extend the first phase of the deal, allowing for more hostage-for-prisoner exchanges.

Mr. Michael said Mr. Trump’s vision for a Gaza without Gazans could work as a threat and put significant pressure on Hamas to release more hostages. Conversely, he said, it could cause Hamas to walk away from the deal altogether.

“Mr. Trump is a businessman,” Mr. Michael said. “He takes risks.”

Zakaria al-Qaq, a Palestinian expert in national security, said that even the mere suggestion of relocating two million Gazans was likely to complicate the cease-fire negotiations by making Hamas more cautious, and to destabilize the entire Arab world.

Mr. Trump’s declaration, he said, was “The perfect recipe for recruiting more people to Hamas,” adding that Mr. Trump’s “new colonialism” had given Hamas “easy marketing tools.”

Many people believe Mr. Trump’s vision for Gaza is not feasible, but regardless of the reality, Mr. Netanyahu has come out with no sign of being pressured by Mr. Trump, or of any daylight between them. His government intact, for now.

An Israeli official who briefed Israeli political reporters in Washington after the Netanyahu-Trump meeting said it was now clear to Mr. Netanyahu’s coalition partners that bringing down his right-wing government with Mr. Trump as president would be irresponsible and foil “historic” opportunities in the coming years.

Relatives of the hostages warn that they do not have time.

“I live in daily fear,” said Alon Nimrodi, the father of Tamir Nimrodi, an Israeli soldier who is slated to be released only under a second phase of the deal.

Mr. Trump’s vision for Gaza was not a bad one, Mr. Nimrodi said. “But this is not the time to talk about it,” he said. Plans for Gaza should wait till “after the hostages are out,” he said.

It was about 12:30 p.m. on a gray Tuesday in Orebro, Sweden, when gunfire erupted at an adult education center, with students and teachers in class. Minutes later, as an alarm wailed, the police charged into the center, Campus Risbergska, where they encountered a chaotic, smoke-filled scene. It would be hours before they gave an all-clear.

“An inferno. Dead people. Injured people. Screams and smoke,” Lars Wiren, Orebro’s police chief, said Thursday at a news conference as he described the scene.

The shooting in Orebro, a quiet university town in central Sweden that has become a haven for migrant communities, has left the country reeling. The national government has described the attack as the worst mass shooting in Sweden’s history, and flags around the country are flying at half-staff. In the days after the attack, the authorities are still seeking to understand why a man entered the campus and opened fire. At least 11 people were found dead, the police said, and the suspect was believed to be among them.

The police have not publicly identified the gunman or shared any details about a possible motive. Swedish news outlets have identified him as Rickard Andersson, a 35-year-old who lived alone in an apartment near the school.

The police have not confirmed the reports, and The New York Times has yet to independently confirm the gunman’s identity. Officials have begun to share how the attack unfolded, describing a heavily armed assailant who worked alone.

When officers entered the school on Tuesday, it was filled with smoke from pyrotechnics, Mr. Wiren said. The police came under a hail of gunfire so intense that officers could not tell how many shooters were on the scene, he added. Through the smoke, officers saw a man approaching them and carrying what appeared to be a rifle.

The man continued to shoot, emptying several magazines of ammunition. In the chaos, none of the 130 police officers chose to return fire, the police chief said.

“You have to respect the situation. There’s panic among the students, the teachers,” Mr. Wiren said. “There’s smoke. The circumstances must be right in order to fire.”

After about an hour, officers later found the gunman among the dead. Near his body were three weapons, including the rifle. Scattered nearby were at least 10 empty magazines and unused ammunition, Mr. Wiren said.

Police officers fanned out across the four-acre campus, searching for victims and possibly more perpetrators in an operation that lasted over three hours, according to a police timeline of events.

In one of the classrooms, Hellen Werme and other students hid for at least two hours under tables and hospital beds used for training. As soon as shots rang out, they locked the door and hid on the far side of the room, she said. At one point, they heard the gunman pass by.

“We were very quiet. He was close,” Ms. Werme, 35, recalled a day later. “I could hear his steps, but then the shots were further and further away.”

The students and their teacher huddled in fear until they heard loud raps on the door, and voices yelling, “Police!”

“When we saw the police I cried tears of joy,” she said. “I just looked at the police and followed them all the way out.”

By late afternoon, the police had enough reason to believe that the suspect in the shooting had been rushed to a nearby hospital, according to an online statement. With the school still under lockdown, they started an investigation that included nearby suburbs, questioning residents.

Police said they have identified the suspect and were awaiting DNA confirmation, Anna Bergqvist, an investigating officer, said on Thursday at the news conference. He had no prior convictions and was not previously known to the police. Investigators have traced four firearm licenses to the suspect, Ms. Bergqvist said.

In the last two days, officers have searched his apartment, seizing computer equipment and telephones. They were also studying hours of video and audio recordings.

“We think the picture is starting to come into focus, but we can’t provide any details yet,” Kristoffer Zickbauer, another investigating officer, said Thursday.

A news outlet, Aftonbladet, reported that Mr. Andersson, the man local media have identified as the shooter, lived an isolated life, alienated from friends and family.