EVERY DAY THERE ARE YOUNG, sometimes tiny human beings who leave the Children’s Hospital of Philadelphia—each one of them living, breathing proof of the miracle work that takes place inside. They are survivors of childhood cancer and rare diseases and congenital conditions. They are alive because CHOP has some of the world’s most capable medical professionals operating with technology on the most far-reaching frontiers of science.

But earlier this month, some of the children inside CHOP were exposed to a potentially deadly threat that medicine learned to stop a long time ago, and that until recently was mostly gone from the United States.

That threat is measles. And it might not be such a threat if Robert F. Kennedy Jr. hadn’t spent so many years undermining faith in vaccines—or if, as health and human services secretary, he was treating the surge in cases like the big deal it is.

Here’s what happened at CHOP, according to the city’s Department of Health and a reconstruction of events by hospital staff: The exposure took place on Friday, September 11, when somebody presumed to have measles was at the hospital for nearly seven hours spanning the morning and afternoon.¹ This person spent some of their time in the cafeteria and an atrium, where the virus could have lingered in the air for more than two hours as literally hundreds of people came through. No less alarmingly, this person with measles likely spent additional time in the neonatal intensive care unit and its welcome lounge for families.

NICUs are where you will see hospitals like CHOP doing some of their most astonishing work, as staff care for severely premature infants and newborns with breathing difficulties, heart disorders, or other serious conditions following birth. NICUs are also where you will find some of the patients most vulnerable to infection, because—among other things—the transfer of maternal antibodies to children mostly takes place during pregnancy’s third trimester. And that’s to say nothing of those babies born to mothers who cannot pass on measles immunity because they have none.²

An unvaccinated person probably brought this threat into CHOP in the first place. Philadelphia deputy health commissioner James Garrow confirmed to me in a phone interview that ...