WASHINGTON ― Esmat Fathi, 28, thought she had done everything she needed to do to start researching Alzheimer’s disease at the University of Memphis in Tennessee this week: She earned a master’s degree, resigned from her job to do applications, got into a Ph.D. program, secured a visa, arranged for a place to stay and bought a flight. But when she checked her cellphone on Monday, she had a message from her friends: Don’t go.

Three days earlier, President Donald Trump had signed an executive order banning nationals of seven Muslim-majority countries, including Iran, from entering the United States for the next 90 days. It was done under the guise of keeping America safe from potential terrorist attacks. But the earliest visible impacts of the policy were felt in the worlds of academia, medicine, business and tech. Fathi, who is Iranian, watched her career arc change abruptly.

“Every morning I wake up and wish it was a bad dream,” she told The Huffington Post.

Fathi was slated to join a lab run by Dr. Ramin Homayouni, whose research is focused on understanding the mechanism underlying Alzheimer’s disease, with the hope of contributing to new therapeutics. Her academic background is perfectly aligned with this work: She did her master’s thesis on Alzheimer’s at the University of Tehran, and results from her research were published in a peer-reviewed journal called NeuroMolecular Medicine. She said she planned to come to the U.S. to continue learning how to “alleviate the pain of these patients.”

Thousands of Iranian students are currently studying in the U.S., and the vast majority are in fields related to science, technology, engineering and math. Most would prefer to stay in the U.S. after graduation to continue their scientific pursuits, according to a study published in 2014 by the Washington Institute for Near East Policy, a think tank that aims to advance U.S. interests in the Middle East.

Trump’s ban has many international students and faculty in the United States worried about whether they will be allowed back in the country if they go abroad for work or to visit family. Others fear that the travel ban — as well as the administration’s open skepticism toward subjects like climate change — will also have the long-term effect of pushing the best and the brightest away from America.

The Association of American Medical Colleges, for example, estimates that roughly 1,000 non-U.S. citizens associated with the seven countries that constitute Trump’s Muslim ban have applied for residencies and fellowships in the U.S. They also say there are more than 10,000 physicians who were born in one of those seven countries. At a time when America already faces a physician shortage, the possibility that the existing universe will be narrowed further ― and that the climate will become even less inviting for those thinking of entering the field ― has officials warning about harmful ripple effects.

“This will exacerbate existing shortages,” said Matthew Shick, director of government relations and regulatory counsel at AAMC. “We are really concerned about the impact it will have on recruitment this year and the next few years. And that will impact some of the most vulnerable populations around the countries that are in underserved communities as a result of existing physician shortages.”

Faced with the possibility of a massive disruption in biomedical research, academic studies and even medical care, scientists and academics have been compelled to engage politically. More than 5,800 international professors and academics have signed an online petition expressing solidarity with people who were affected by the ban. Officials at various science consortiums have urged their groups to stop holding conferences in the United States. And nearly 50 college and university presidents sent Trump a letter, urging him to “rectify and rescind” the order.

Although the Trump administration has tinkered with the ban to make it less cumbersome for green card holders and those helping out U.S. military officials overseas, there is scant evidence that it will back down more significantly. And so, students and scientists who had plans to pursue their studies in the United States have been forced to reassess.

One of them is Mahdi Shahabadi, Fathi’s friend who warned her not to get on a plane. Shahabadi is also a Ph.D. student and loves watching TED Talks — particularly ones by evolutionary biologist Richard Dawkins and neuroscientist Sam Harris — and was accepted to do research at the University of Wyoming on advancing technologies related to water treatment. He was also supposed to start his studies this week.

But he says that a U.S. officer stopped him and about a dozen other people when he tried to board his flight to the United States over the weekend. The officer told them about Trump’s executive order, and Shahabadi returned to Iran.

“The whole situation was humiliating,” he said. “When you come back to your country and call your mum, and hear her crying for your ruined career and wasted energy, time and money, who is responsible for that? I’m asking this question every second.”

Shahabadi added that he believes the ban “only benefits the extremists in both sides, and we ordinary people are the very victims.”

When you come back to your country and call your mum, and hear her crying for your ruined career and wasted energy, time and money, who is responsible for that? Mahdi Shahabadi

At home in Tehran, Fathi finally unpacked her suitcases on Thursday. She doesn’t know if she will be able to apply for another visa, or if she will lose her spot at school, although a University of Memphis spokesperson told HuffPost that the school is committed to working with all students and faculty affected by the ban and “ensuring they continue in their academic programs.”

Both Fathi and Shahabadi are trying to get to Boston because a court order temporarily halted Trump’s executive order from taking effect there until at least next week. The president’s attorneys are fighting the order. Shahabadi said he can only go if it is extended. Fathi hasn’t been able to get a plane ticket.

Even if she gets another chance at entering the United States, she’s not sure she can afford it. Visiting a U.S. embassy again — she has to travel to a different country to get to one — would cost her about $1,200 in travel and fees, she said, and she already had to borrow money from friends and family the last time.

“You can’t imagine how it feels when you can’t continue your education because of your nationality,” she said. “It’s really devastating.”

UPDATE: Feb. 4 ― Because a federal judge in Seattle temporarily blocked the travel ban on Friday, Fathi is once again planning to come to the United States. She’s due to arrive in Detroit on Monday.

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