Facing a rapidly spreading outbreak of measles, Mayor Bill de Blasio has rightly declared a public health emergency, with fines for those who refuse to be vaccinated.

With 285 cases already documented here this year (versus just two in all of 2017), the city is requiring everyone in Williamsburg to get the measles vaccines. Those who fail to comply could face fines up to $1,000.

This follows an earlier threat to close yeshivas that refuse to bar unvaccinated students.

The outbreak is centered in ultra-Orthodox Jewish communities where vaccination rates are far below normal, due to some parents’ objections. So the city’s response touches on questions of religious and parental rights, gray areas where governments rightly tread lightly.

Indeed, Gov. Andrew Cuomo was quick to declare the city’s action “legally questionable” and “a serious First Amendment issue” — though his failure to suggest any practical alternative suggests this was mostly his usual automatic dumping-on-de Blasio.

Nor is the city alone in recognizing the problem: Rockland County last month banned the unvaccinated from public spaces for 30 days. But a judge last week overturned it, saying it’s not yet a clear emergency.

Public health officials insist otherwise. And the US Court of Appeals in 2015 upheld the city’s mandatory vaccination law, ruling that exclusion of religious exemptions “during an outbreak of a vaccine-preventable disease is clearly constitutional.”

We’ve long been strong defenders of individual religious rights, and we remain reluctant to support intrusive government coercion. But there’s no right to allow your child to infect others with a serious disease.