Every child older than 6 months should get a flu vaccine. That’s according to the Centers for Disease Control and Prevention. The American Academy of Pediatrics agrees. The science is clear: Vaccination against flu saves children’s lives.

Meanwhile, in the past year three migrant children have died of illnesses related to influenza in U.S. government custody. With flu season here, will the government follow its own recommendations and vaccinate the children in its care? Earlier this year a group of public-health experts asked that question in a letter to lawmakers. U.S. Customs and Border Protection gave its answer: No.

To understand how quickly disease can spread in detention centers, consider this: More than 50 camps on the southern border have had mumps outbreaks over the past year, according to a CDC report released in late August. Of the nearly 900 migrants who became ill, the vast majority caught the virus in the camps. So did several dozen employees.

This was entirely predictable. Migrant centers are prone to outbreaks for the same reason prisons, college dormitories and cruise ships are — when lots of people live close together, germs have a chance to mingle. That’s true in even the best-run facilities, which is why so many U.S. colleges require vaccinations before move-in day. Migrant detention centers aren’t so pleasant.

By most accounts, the adult camps are overcrowded and unsanitary. The children’s facilities appear to be not much better: Until recently, President Donald Trump’s administration was arguing they didn’t necessarily need soap, toothbrushes and showers. Children have reported going hungry; sleeping on the floor in the freezing cold; and living amid open-air toilets, lice and mosquitoes. A flu outbreak this year at a Texas camp left five babies hospitalized in intensive care.

The agency has said it doesn’t vaccinate children “due to the short-term nature of CBP holding and the complexities of operating vaccination programs.” That’s a cop-out. Detention centers routinely hold children for a week and a half or more, according to a Homeland Security report from July, and the administration has said it favors indefinite detention. And about those administrative complexities: The flu vaccine typically comes in a single shot.

FROM AN EDITORIAL ON Bloomberg OPINION