An executive order signed by President Donald Trump on Friday bars citizens of seven countries from entering the U.S. for 90 days. An estimated 218 million people from majority-Muslim nations are affected by the ban, according to data from the World Bank. North Texas doctors and scientists are concerned that the policy could have far-reaching effects for the health of Texans.

Texas has a doctor shortage. The state has 177 doctors for every 100,000 people, compared with the national average of 236 doctors, according to a report published in 2015 by the North Texas Regional Extension Center. That ratio puts Texas at No. 45 on the national list of doctors per person.

The state is not producing enough doctors to keep up with its growing and aging population. Trump's executive order could worsen the doctor shortage.

Texas is "in the vulnerable position of depending on other states and countries to produce the physicians we need." — Texas Medical Association

When you see a doctor in Texas, there's a good chance they didn't train in the U.S. One in 5 doctors here went to medical school outside the U.S. or Canada.

An investigation by the Texas Tribune found that more than 1 in 4 doctors who receive a license to practice medicine in Texas come from another country. Many are recruited by hospitals and government agencies, such as the Department of Veterans Affairs, to work in rural and medically underserved communities across the state.

It’s hard to know exactly how many Texas doctors hail from the seven countries listed in Friday’s executive order.

In a memo to staff at the University of Texas at Austin, President Greg Fenves said: “We have 110 students, faculty members and scholars who are citizens of the seven affected countries — Iran, Iraq, Libya, Syria, Yemen, Somalia and Sudan. The talents that brought them to UT are deeply valued, and their perspectives represent an essential part of the university.”

But beyond the countries listed in the ban, Trump's policy sends a “chilling message” to doctors and scientists across the Arab world, said Dr. Peter Hotez, a pediatrician and professor at Baylor College of Medicine in Houston.

“My concern is that any doctor or scientist from an Arab or Muslim country will now think twice about coming to the U.S. for postdoctoral study or a faculty position," he said. "I’m concerned that two months from now, the ban will extend to other countries in the Middle East.”

Hotez and his team develop vaccines for neglected infections. They recently entered a collaboration with King Saud University in Saudi Arabia. While Saudi Arabia is not mentioned in the executive order — allegedly because of the president's business ties in the country — Hotez is fearful about the future of his research.

“I’m assuming everything with our work in Saudi Arabia is still a go, but if this gets virulent and angry, who knows where it could end up,” he said.

Suna Burghul, 22, is a first-year medical student at the Texas College of Osteopathic Medicine. A Palestinian-American who was born in Tyler to a refugee father, Burghul is not affected by the immigration ban but worries it is adding fire to growing anti-Muslim sentiment.

“I’m scared for so many people and not just in the countries he put the executive ban on, but people who even look like they could be Muslim,” she said.

While shadowing in a hospital when she was in high school, Burghul listened as a surgeon made racist, Islamophobic remarks.

“We were standing in the [operating room], and there was a conversation about a bridge, and he said, ‘I’m surprised the turban heads haven’t blown it up already.’ I didn’t know what to say.”

Burghul wrote about her faith in her medical school application essay and said she has always felt proud to be a Muslim. That has changed recently, she said.

"I even mentioned [being Muslim] in my medical school interviews, and now I'm like, should I hide it in case it will affect my career? I worry what people will think if they know I'm Muslim."— Suna Burghul, first-year medical student, Texas College of Osteopathic Medicine

The effects of the immigration ban are already being felt by doctors and scientists hoping to work in the U.S. An Iranian scientist living in Switzerland was headed to Boston to work at the Harvard-affiliated Brigham and Women's Hospital on Saturday when an American government official stopped her from boarding her connecting flight in Germany. Samira Asgari planned to study genetics and infectious diseases.

A Brooklyn doctor is stranded in Sudan, unable to return to his patients at Interfaith Medical Center in New York because of the ban.

Another doctor on her way to begin an internal medicine residency at the Cleveland Clinic was banned from entering the country because she held a Sudanese passport. Suha Abushamma, 26, was told by officials in New York that she could voluntarily leave or be deported.

"I'm only in this country to be a doctor, to work and to help people — that's it," she told a reporter at ProPublica. "There's no other reason."

Hotez said those stories are just the beginning of what could be a brain drain in the U.S.

“Scientists from other countries have options about where they want to work," he said. "They can go to the U.K., Germany, Singapore, all of these places doing enormously good science. This will have a negative effect on science in the U.S.”