A prominent Asian virologist with longstanding ties to the United States was making plans to attend a conference here when he learned he couldn’t get a visa. It was the first time he had been blocked from visiting after decades of trips to collaborate with American researchers. And the timing couldn’t be worse. His expertise is critical as scientists struggle to get a grip on the coronavirus pandemic.

“Curtailing his engagement with the global scientific community is the kind of isolation we don’t need right now,” said Robert T. Schooley, an infectious diseases specialist at the University of California San Diego, who — in order to protect the virologist — declined to identify him. “It’s in the world’s best interest to have open science, especially at times like these.”

This isn’t an isolated case. The Trump administration is targeting scientists in its crackdown on foreign influence, worried that researchers from abroad will steal U.S. secrets and jeopardize national security. The administration is denying visas or restricting them, not just to established scientists but to students who want to study here. Just this week, President Trump announced plans to temporarily suspend immigration to the United States. All this is happening during a tremulous time, as the world confronts a dangerous virus.

“It is an embarrassment for our country,” Schooley said, speaking about his colleague who is unable to return. “His experience in virology would be vital in controlling this coronavirus. He is the type of person we should be actively seeking to recruit to migrate to the United States, rather than kicking to the curb.”

Federal agencies, including the National Institutes of Health (NIH), the National Science Foundation (NSF), the Departments of Energy and Defense are scrutinizing foreign-born researchers. The Trump administration is taking aim at China, in particular, saying it’s trying to prevent Chinese scientists from stealing U.S. intellectual property.

Former secretary of energy Steven Chu spoke about the administration’s stance at a recent meeting of the American Association for the Advancement of Science (AAAS), saying, “Restricting international scientific collaborations with China and the non-approval of student visas of Chinese PhD students is not in the best interests of America.”

While he endorsed efforts to protect U.S intellectual property, Chu, the current president of AAAS, said that curbing collaboration with Chinese scientists stymies U.S. research. He described PhD candidates at Stanford and UC Berkeley who had been blocked from reentering the United States, saying that their denial will discourage other students from coming to the United States to study and do research.

Chu is himself the son of Chinese immigrants who studied at MIT. He is also a Nobel Prize winner.

In addition to blocking foreign-born scientists from entering the country, federal officials are also examining grant applications to ensure researchers from abroad are not being paid by a foreign government, including their own. The administration fears that scientists receiving foreign income will share valuable U.S. research with foreign actors, but U.S. researchers who work with these scientists say the excessive scrutiny is overly intrusive, and complain that it unfairly targets the Chinese.

“One of my Chinese colleagues, who has been with me for 15 years, is constantly being harassed by the government,” said Robert C. Gallo, who co-discovered HIV in 1984, and now heads the University of Maryland’s Institute of Human Virology. “They make him go through hoops with every grant he submits,” Gallo said. “It’s happening much more now than it ever did before.”

Andrew Rosenberg, director of the Union of Concerned Scientists’ center for science and democracy, said he understands concerns about intellectual property theft, but he believes the administration’s efforts would do little to prevent it.

He also believes suspicions of foreign-born scientists are based more on racism — focusing on China — than on any recent acceleration of espionage. He said that by making it harder for talented scientists to come here and do research, the Trump administration is hurting more than helping.

“We have been concerned about theft since the Cold War, but if somebody wants to get in and steal secrets, it seems to happen anyway, no matter what we do,” he said. “There is no sense in banning scientists from a particular country or making it difficult to come here. They think just because they look different from us they must be dangerous. Using these broad brush approaches doesn’t work.”

William Kerr, a professor in the Harvard Business School and author of The Gift of Global Talent, said that the administration’s hostile attitude toward immigrants more generally also discourages foreign scientists from coming to the United States.

Since taking office, Trump has put in place a series of restrictive immigration policies, from his travel bans on citizens from predominantly Muslim countries to his policy separating migrant families at the southern border. Such initiatives can inspire fear in would-be immigrants.

“Images of children in cages can substantially dampen someone’s vision of America as a welcoming place,” Kerr said. “The important question people face when thinking about migration is, ‘Is this a place I want to make a long-term home? Do I want to build a career there?’” he said. “Will it be a society that welcomes people or becomes more hostile?”

Trump’s stance on immigration isn’t just frightening foreign-born researchers. It is also worrying American scientists who work with them.

“I couldn’t do anything here without these people,” Gallo said of his team, which includes scientists from China, England, India, Holland, Spain, Italy and Iran. “Their research keeps us going. I understand the concern about technical theft, but it’s so rare in medicine. I never hear of theft in biology or medicine.”

Last fall, AAAS sent a letter signed by 60 medical, scientific and technical organizations to the directors of the NSF, NIH, the Defense and Energy Departments, and to Kelvin Droegemeier, who directs the White House Office of Science and Technology Policy. In it, they warned that the administration’s approach to immigration would slow scientific progress.

Foreign-born researchers have contributed much to the advancement of medicine, basic science and technology in the United States. ARPANET, the precursor to the internet, for example, was developed, in, part, by scientists from Wales and Poland, while the Apollo project involved researchers from Cuba and Egypt, among other places. In the United States, scientific advancement has always depended on researchers from across the globe. Naturalized immigrants account for more than a third of all Nobel prizes that Americans have won for chemistry, medicine and physics.

Rosenberg said that, right now, the United States needs foreign researchers more than ever. While he understands the need for current bans on travel from Europe and elsewhere to curtail the spread of the coronavirus, he also feels such bans shouldn’t apply to researchers who want to work with U.S. scientists to develop a vaccine against the virus or drugs to treat it.

“You don’t want people moving into an area where there are wildfires,” he said. “But you do want the firemen to be able to get in.”

Marlene Cimons writes for Nexus Media, a nonprofit climate change news service.