Mumps has swept through 57 immigration detention facilities in 19 states since September, according to the first U.S. government report on the outbreaks in the overloaded immigration system.

The virus sickened 898 adult migrants and 33 detention center staffers, the Centers for Disease Control and Prevention said in its report Thursday.

New cases continue as migrants are taken into custody or transferred between facilities, the report said. As of last week, outbreaks were happening in 15 facilities in seven states.

In response to the report, Immigration and Customs Enforcement spokesman Bryan Cox said medical professionals at detention facilities screen all new detainees within 24 hours of their arrival to ensure that highly contagious diseases are not spread.

Cox said some detainees come from countries where communicable diseases are less controlled than in the U.S. and carry with them the risk of spreading infection.

The CDC report said more than 80% of patients were exposed while in custody. Mumps is a contagious virus that causes swollen glands, puffy cheeks, fever, headaches and, in severe cases, hearing loss and meningitis.

In the U.S., vaccines have drastically reduced the number of mumps cases. Only a few hundred cases are reported most years, with periodic outbreaks involving colleges or other places where people are in close contact.

In the migrant center outbreaks, at least 13 people were hospitalized, the CDC reported.

A large portion of the cases have been in Texas. The Texas Department of State Health Services raised the alarm in December, followed by six other state health departments in early January, prompting what the CDC report calls “a coordinated national outbreak response.”

ICE has given more than 25,000 doses of measles-mumps-rubella vaccine in the affected facilities.

The CDC did not identify detention facilities, but said 34 of them are operated by private companies. The report said migrants were being held in 315 facilities in mid-August.

Nashville immigration attorney R. Andrew Free has been tracking facilities with mumps outbreaks from reports of advocates and lawyers representing detainees.

“This has all the makings of a public health crisis,” Free said. “ICE has demonstrated itself incapable of ensuring the health and safety of people inside these facilities.”

An influx of migrants at the U.S.-Mexico border earlier this year has taxed the immigration system. The CDC report dealt only with mumps, not other health problems in detention facilities. At least two migrant children have died of complications of the flu after being detained by U.S. Border Patrol.

The CDC report said detention facilities should follow guidance from state and local health departments when responding to mumps.