Physicians from two top universities called on Congress to promptly investigate the deaths of six migrant children in the Trump administration's custody, warning that the widely-reported poor conditions in detention centers could be putting thousands of children at risk for infectious diseases.

Three of the six children who died while in U.S. custody since last September died partially because they had contracted influenza, the doctors noted. Two died after spending twice the amount of time in a Border Patrol station than the Department of Homeland Security (DHS) typically allows.

It's "necessary to better understand the conditions at the border to avoid more deaths," tweeted Paul Spiegel, a public health professor at Johns Hopkins University.

We wrote this together w our Harvard colleagues - necessary to better understand conditions at border to avoid more deaths @humanit_health @drjosh https://t.co/0TdJOpoXIb — Paul Spiegel (@pbspiegel) August 1, 2019

The administration "may be overlooking the risks of outbreaks that are entirely preventable," Joshua Sharfstein, another Johns Hopkins professor, told the Washington Post.

About 2,000 migrant children are in U.S. custody on any given day, many after a grueling journey from Central America and elsewhere. The children are in Border Patrol stations, where many have been kept for far longer than the facilities are equipped to house people; detention centers run by ICE and the Health and Human Services Department (HHS), and facilities run by for-profit government contractors like CoreCivic.

The doctors urged lawmakers to probe how the widely-reported unsanitary conditions in the centers—detailed by children in a facility near El Paso in June—contributed to the deaths that have been recorded.

Overcrowding has forced dozens of people to stay together in a single room, often with insufficient blankets. Some have slept on restroom floors or outside and many have reported that they were unable to bathe while in custody. A Department of Homeland Security (DHS) spokesperson told the Washington Post that children are not vaccinated against the flu when they are first detained, but they are given vaccines when they are transferred to HHS.

In June, CBS News reported that a dozen children in a facility in Clint, Texas, were exhibiting signs of the flu.

"With so many lives at risk, these issues are worthy of congressional investigation. Another influenza season is around the corner, and there are other types of infectious diseases that pose a threat to detained populations," wrote the doctors. "Timely action is critical."

The letter was sent a day after a woman named Yazmin Juárez filed a lawsuit against CoreCivic, the for-profit prison company which was awarded a contract by the Trump administration to house and transport migrant children. Juárez's one-year-old daughter, Mariee, died last year, weeks after being released from a center run by the company.

The child developed a respiratory illness and a fever of 104.2 degrees after being transferred with her mother to CoreCivic's detention center in Dilley, Texas. Juárez says the staff at the center didn't give Mariee proper medical treatment and then cleared her to travel when it wasn't safe.

"We don't believe that it's ever appropriate to jail small children," Stanton Jones, a lawyer for Juárez, told PBS Newshour. "At a minimum, if CoreCivic is making huge amounts of money to run a jail for children, there are legal duties that come with that."

The three children who exhibited signs of the flu died between December 2018 and May 2019. Before the six recent deaths over the past year, it had been a decade since a migrant child died in U.S. custody.