The herpes simplex virus, common and relatively harmless in adults, can be deadly to babies. Such infections in newborns are blessedly rare, but one thing is known to increase the risk significantly: the circumcision ritual called metzitzah b’peh, practiced by many ultra-Orthodox and some Orthodox Jews, in which a circumciser, or mohel, sucks blood from a newly cut penis with his mouth.

The New York City Health Department, American Academy of Pediatrics, Centers for Disease Control and Prevention and other authorities have long warned about the dangers of the practice. The Health Department has linked it to more than 12 herpes cases, and two deaths, since 2000, and has tried to get mohels to stop doing it.

That is why it is distressing to see Mayor Bill de Blasio and Orthodox leaders celebrating a deal they made this week to abandon the city’s modest effort to regulate the practice, and instead leave it to the ultra-Orthodox community to help limit the damage metzitzah b’peh does — but voluntarily, and only after babies get sick, not before.

Mr. de Blasio wants to stop requiring parents to sign a waiver acknowledging the risks of metzitzah b’peh. Rabbis called that policy, begun under Mayor Michael Bloomberg, an unconscionable infringement on religious freedom. They refused to use the forms and sued the city.