Immigrants held in U.S. detention centers have been particularly vulnerable to the spread of communicable diseases — including thousands who were put under quarantine last spring for mumps, measles, flu and other illnesses — and it is unclear whether the coronavirus could pose a serious concern for U.S. authorities and the tens of thousands of foreigners in their custody.

Immigration and Customs Enforcement detains nearly 38,000 people in more than 130 private and state-run jails and prisons across the country, many of which sit in rural areas and operate with minimal public oversight.

Jenny Burke, an ICE spokeswoman, said Thursday that aspects of the agency’s pandemic workforce protection plan, developed in 2014, have been in effect since January to prevent and mitigate the spread of the novel coronavirus among the detainee population and staff.

She said that since reports of covid-19 began, ICE epidemiologists “have been tracking the outbreak, regularly updating infection prevention and control protocols, and issuing guidance to ICE Health Service Corps staff for the screening and management of potential exposure among detainees.”

ICE officials said that as of March 3, four detainees had met the criteria for coronavirus testing, but none has tested positive. The number of confirmed cases across the United States has jumped from a few dozen to more than a thousand since then, but the agency declined to say whether any more detainees have been tested as infection numbers climbed nationwide.

Immigration advocates say they are concerned about the potentially devastating impact a coronavirus outbreak could have inside the U.S. government’s crowded immigration jails.

The American Civil Liberties Union of Southern California and Human Rights Watch this week called on the Trump administration to develop a strategy to prevent and mitigate such an outbreak at facilities that have long been plagued by allegations of detainee abuse and inadequate medical care.

“People in detention are highly vulnerable to outbreaks of contagious illnesses. They are housed in close quarters and are often in poor health,” the groups wrote in a letter to ICE officials overseeing an ICE processing center in Adelanto, Calif. “Without the active engagement of the detention center’s administration, they have little ability to inform themselves about preventive measures, or to take such measures if they do manage to learn of them. We are particularly concerned about the health and safety of the people detained at Adelanto, given the facility’s demonstrated failure to provide adequate medical care in the past.”

The groups sent similar letters to other ICE detention facilities, calling on ICE to educate detainees and staff about proper hygiene measures to reduce the spread of coronavirus in ICE facilities; provide sufficient supplies for hand-washing and cleaning; develop plans to screen and test for the virus, as well as contain and treat the infected; and develop staffing contingency plans for the possibility that significant numbers of ICE detention staff will fall ill from the virus.

ICE officials said the agency has 20 detention facilities run by its Health Service Corps, including 16 that are equipped with airborne infection isolation rooms, where officials said they plan to house detainees deemed at risk for covid-19 or displaying symptoms. Officials said ICE detention staff also have received guidance on the use of protective equipment.

During a surge of border apprehensions last spring, ICE placed between 4,000 and 6,000 detainees in quarantine as a result of mumps, measles, flu and other communicable disease outbreaks, ICE’s acting director, Matthew Albence, told lawmakers during a congressional hearing Wednesday.

“We have extensive experience with regard to keeping them isolated so that doesn’t spread, and we also have extensive medical experience,” Albence said, citing the agency’s medical staff and Public Health Service Corps personnel.

Government investigators and attorneys have previously reported serious patterns of neglect at several ICE facilities across the country, including denied or ignored requests for medical care, limited dissemination of vaccinations for the flu and other infectious diseases, poorly enforced or mismanaged quarantines, and overcrowding.

Ranit Mishori, a senior medical adviser for Physicians for Human Rights, said she was concerned about ICE’s capacity to both test detainees and quarantine them when necessary.

“I had to turn away patients yesterday who may have the mild forms of covid-19,” said Mishori, who practices family medicine in Washington, D.C., citing federal guidelines that restrict the conditions under which coronavirus testing can be authorized. “If we are not equipped throughout the country to test the people who need to be tested, imagine what happens in these facilities to these marginalized populations.”

ICE has reported at least 21 deaths in its facilities over the past two years — eight of which occurred in the past six months alone — including, most recently, a 22-year-old female Guatemalan asylum seeker who died of hepatitis, septic shock and liver failure, after being detained at a Texas facility for more than six months; and a 63-year-old man held in Florida who died in January after weeks of respiratory distress and other serious symptoms of illness. In December, a 40-year-old, also held in Florida, died after he succumbed to septic shock.

Advocacy groups and Democratic lawmakers have urged ICE to reduce the number of people in its custody by releasing particularly vulnerable people — many of ICE’s detainees are asylum seekers with no previous criminal record — to minimize the crowded conditions that facilitate the spread of the disease and ensure that people with underlying conditions get access to care.

Albence told lawmakers Wednesday that his agency will not release vulnerable detainees as a preventive measure against coronavirus: “The people that we have in detention are there because they are public safety threats or flight risks.”

On Thursday, the union that represents immigration judges across the country called on the Justice Department to suspend hearings in a wide swath of cases out of fear that such proceedings could help spread the coronavirus.

In a letter to the director of the Justice Department’s Executive Office for Immigration Review, Judge Ashley Tabaddor said that all of the “non-detained master calendar dockets” — essentially, hearings for people not in federal custody — “must be completely suspended because they present a continuing and unacceptable risk in the current environment.”

Tabaddor, the head of the National Association of Immigration Judges, said the dockets typically bring 50 or more respondents into each courtroom, many of whom have recently traveled internationally, plus attorneys and other family members. She said that the union had been informed Wednesday that one attorney in a March 10 hearing reported having flu-like symptoms and was concerned about a coronavirus infection.

“This is exactly the type of situation the White House, CDC and other public health authorities urge us to avoid, and we trust that you will agree that it is untenable and irresponsible in light of the current spread of COVID-19 infections across the country,” Tabaddor wrote.

The association that Tabaddor represents also recently publicized orders that judges received from the Justice Department to take down courtroom signage on methods to prevent the spread of coronavirus. The department then reversed that order.

The association also has called repeatedly on the Executive Office for Immigration Review to provide its judges more clarity on how they can manage their courtrooms to prevent the spread of covid-19, and as of Thursday, said they had received no response.

Immigration attorney Matthew Archambeault said immigration judges in Newark on Thursday allowed lawyers to attend court hearings while their clients waited outside to minimize close contact. But that, he said, left the waiting room packed with dozens of people.

“I’m worried about myself; I’m worried about my family,” Archambeault said. “I have my mother-in-law at home, who is elderly, and she’s not in the best of health.”

A spokesperson for the Executive Office of Immigration Review said the courts’ status “remains the same, and we do not plan any mass closure of immigration courts.”

“As concerns are raised, we will address them on a case-by-case basis as necessary and appropriate,” the spokesperson said.

U.S. Customs and Border Protection, which includes Border Patrol authorities, also grappled with allegations of harsh conditions, and inadequate sanitation and medical care for migrants detained in CBP facilities along the U.S.-Mexico border, particularly during last year’s surge in border crossings, when border infrastructure was overwhelmed.

A CBP spokesperson said border agents are working to identify “individuals who have traveled from or transited through countries affected by COVID-19.”

“Individuals apprehended by U.S. Border Patrol between the ports of entry with symptoms of illness are referred to the CDC or local health officials for additional health screening,” the spokesperson said. “U.S. Border Patrol takes all necessary precautions to ensure that no communicable diseases are spread across populations in custody.”