One day in the early sixties, Saul Zucker, a pediatrician and anesthesiologist in the Bronx, was treating the child of a New York assemblyman named Alexander Chananau. Amid the stethoscoping and reflex-hammering of a routine checkup, the two men got to talking about polio, which was still a threat to the nation’s youth, in spite of the discovery, the previous decade, of a vaccine. At the time, some states had laws requiring the vaccination of schoolchildren, but New York was not one of them. In his office, on the Grand Concourse, Zucker urged Chananau to push such a law, and shortly afterward the assemblyman introduced a bill in the legislature. The proposal encountered resistance, especially from Christian Scientists, whose faith teaches that disease is a state of mind. (The city’s health commissioner opposed the bill as well, writing to Chananau, “We do not like to legislate the things which can be obtained without legislation.”) To mollify the dissenters, Chananau and others added a religious exemption; you could forgo vaccination if it violated the principles of your faith. In 1966, the bill passed, 150–2, making New York the first state to have a vaccination law with a religious exemption. By the beginning of this year, forty-six other states had a version of such a provision; it has proved to be an exploitable lever for people who, for reasons that typically have nothing to do with religion, are opposed to vaccination. They are widely, and disdainfully, known as anti-vaxxers.

Saul Zucker died in June, five months short of his hundredth birthday. Less than two weeks later, the New York Legislature voted to remove the religious exemption, after a contentious debate during which anti-vaxxers harangued from the galleries. Governor Andrew Cuomo signed the bill that night. Following all this on a live stream was Howard Zucker, Saul’s son. Zucker is a doctor—a pediatrician and an anesthesiologist, like his father, and a cardiologist—as well as a lawyer. He is also New York State’s commissioner of health. For more than six months, he’d been at the forefront of an effort to beat back the anti-vaccination movement, as a result of a measles outbreak in the state. Its severity had goaded politicians to change the law, with his support. Because of the success of the anti-vaccination movement, measles cases have since turned up in twenty-nine other states, but New York has had by far the most cases: 1,046 as of last week, out of a national total of 1,203. This has threatened to wind back decades of success in the containment of the disease since the first measles vaccines were introduced, in 1963—an era when the United States saw between three million and four million cases a year. In 2000, the U.S. declared that measles had been eliminated in the country; if this outbreak isn’t contained by October, it could jeopardize the nation’s so-called measles-elimination status. This would be a dire step back for our public-health system, and a national embarrassment. (Britain, well acquainted with national embarrassment, lost its elimination status this year.)

The outbreak began last October, in the Rockland County village of New Square, an enclave of roughly eighty-five hundred Hasidim founded in 1954, on an old dairy farm, by Grand Rabbi Yaakov Yosef Twersky and his followers, who had moved there from Williamsburg, Brooklyn. (Like the many other ultra-Orthodox hamlets that have sprouted up in the area, it is technically in the town of Ramapo.) Twersky’s sect originated in the Ukrainian town of Skvire; when the village in New York was incorporated, Skvire became “Square.” His son David Twersky, who has been the Grand Rabbi since 1968, lives in a house that abuts the New Square synagogue. On religious holidays, Skvire Hasidim come from all over the world—New Square has fifteen sister cities—to worship with him. Although you wouldn’t be wrong to say that New Square is a small, insular monoculture, in epidemiological terms it has the characteristics of an international city.

One such traveller, a fourteen-year-old boy from Israel, became Patient Zero in the state’s largest measles outbreak since 1992. On October 1st, in observance of Simchat Torah, he attended services at the synagogue, for the fifth time in four days. The shul is more than twenty-two thousand square feet and holds seven thousand people, and the bleachers that ring the inside of the building from floor to ceiling were full, as was the gallery upstairs, where the women sit. Feeling ill, the boy left the synagogue and walked up the hill with his father to the Refuah Health Center, which has been delivering medical services to the community since 1993. Most of the clinicians there had never seen a measles case, but they had observed, for a decade, the growth, among their patients, of misgivings toward vaccines. The boy had the telltale rash. Refuah administrators, even before the blood work had come back, notified the county department of health, which advised them to isolate the patient and shut down the health center.

It wasn’t hard to determine where the measles had come from. The boy had caught it in Israel. The theory was that he’d got it from another Israeli, who had travelled to the city of Uman, in Ukraine, for the Rosh Hashanah pilgrimage known as the Hasidic Burning Man. Because of a low vaccination rate, there have been more than fifty thousand cases of measles in Ukraine in the past year. Patient Zero had not been fully vaccinated, but not because of any objection on his parents’ part. In Israel, which is experiencing a measles outbreak of its own, vaccinations are administered in school, and, according to a patient advocate at Refuah, on the day the boy’s classmates had received their shots for measles, mumps, and rubella (known as M.M.R.) he was home sick. The boy’s twin brother, and the rest of his family, had been vaccinated.

It was harder to figure out, in a necessarily timely manner, who’d been exposed. The state and county health departments sent a pair of epidemiologists to New Square—both of them male, out of deference to Hasidic customs of gender separation. The state’s man was Robert McDonald, a doctor and Epidemic Intelligence Service officer from the Centers for Disease Control and Prevention, who had embedded with the state for two years and had, with Zucker and others in the health department, dealt with various recent crises, such as drug-resistant fungus and a hepatitis-C outbreak. McDonald began working on a so-called line list of anyone who might have shared airspace with the boy. He started at the synagogue, where he was greeted by Yitzchok Sternberg, a rabbi with the Khal Mishkan Yosef congregation, whose wife, Chanie Sternberg, is the C.E.O. of Refuah. McDonald drew a map of the interior of the synagogue and set about learning where the boy had been and when, and who else might have been there, too.

Earlier this summer, I visited the New Square synagogue with Rabbi Sternberg, a wry and genial fifty-eight-year-old with a reddish beard. The interior of the shul features arched windows, chandeliers, and a linoleum floor. Rows of tables and plastic chairs face an ornate wooden pulpit, or bimah. (A new, much bigger temple is being built on an adjacent lot. “The day we drove in the last nail on this one, it was too small,” Sternberg said.) Morning prayers were winding down; Rabbi Twersky’s grandson had got married there the night before.