At Clarkstown Pediatrics in Nanuet, New York, babies are on an accelerated measles vaccination schedule, getting their first shots six months early and their second dose right away.

It’s part of a statewide effort to stop several outbreaks of measles from turning into an epidemic. The state has had 170 cases of the highly infectious virus since September, making it the worst year for measles since the 1990s.

Pockets of unvaccinated children have provided fertile ground for the measles virus to take hold. Although measles was eliminated in the U.S. the virus has been brought back by travelers to Israel, which has been battling an epidemic of measles for months. The victims: mostly members of close-knit Orthodox Jewish communities across the state.

“It’s a clear and present danger right here in our community,” said Dr. Douglas Puder, a pediatrician at Clarkstown Pediatrics. His practice is right in the middle of the biggest outbreak, in New York’s Rockland County. As of Jan. 10, county reported 108 cases of measles since the fall. More than 80 percent on average had not been vaccinated and just three cases had received both recommended doses of measles vaccine.

Local, state and federal health officials are battling to fight the outbreaks, which they believe have been fueled by a combination of anti-vaccine propaganda, lax enforcement of school requirements to vaccinate, and a growing trend among some families to turn away from standard medical care. As a result, some private schools in the county reported vaccination rates had fallen to as low as 50 percent. It takes a vaccination rate of 95 percent or higher to prevent outbreaks of disease.

Thus, the full-court press to get children vaccinated. “We have made an incredibly aggressive effort to address this,” said New York state health commissioner Dr. Howard Zucker. “This has been the worst measles outbreak in recent history in New York state.”

More than 13,000 people, mostly young children, have been vaccinated over the past three months at state and county clinics. More than 40,000 educational flyers have been distributed to homes in the hardest-hit communities, and more than two dozen schools and daycare centers closed because of low vaccination rates. Unvaccinated children have been required to stay home for their own protection until they get the shots.

“To date there are more than 30 schools, daycares, and nursery schools in Rockland county that have been required to enact exclusion policies to help stop the spread of measles as permitted by New York State Public Health Law,” the county says on its website.

And individual clinics are making their own efforts.

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“We have brought in every child six months and older to give them an MMR (measles, mumps and rubella vaccine),” Puder told NBC News.

A child usually receives the first MMR vaccine at the age of one year, and a second dose by age four. But to protect local kids in the face of a spreading, highly contagious virus, the schedule has been accelerated. “We have been bringing the one-year-olds back at 13 months to get a second dose,” Puder said. “It’s up to us to keep this from spreading. This could become a truly major epidemic.”

It took years for the pile of dry tinder to build. Three things have to happen to before a measles outbreak can take hold in the U.S., where homegrown measles has been eliminated: A population must have substantial numbers of unvaccinated or under-vaccinated people; some of those unvaccinated people must travel to an area where measles is spreading and then must themselves get infected and bring it back.

Health officials have struggled to get a handle on how this happened. Statewide, New York has good vaccination rates. More than 92 percent of children in New York have received at least one dose of the MMR vaccine, according to the Centers for Disease Control and Prevention.

But New York is one of 47 states that still allow parents to refuse to vaccinate their children for religious reasons, and confusion over that may have led some parents to opt out, Zucker said. That’s even though no organized religious group forbids its member to vaccinate children, and Jewish law specifically instructs followers to protect their own health as well as the health of their children and community.

“I have been out to the community many times now and met with the rabbis there,” Zucker said. He said religious leaders have been puzzled by some of the vaccine skepticism and are advising their congregations to vaccinate.

One issue: Private schools don’t necessarily follow state guidelines for vaccinating students as a prerequisite for enrollment, and Rockland County has a large number of private schools, said John Lyon, a spokesman for the county.

“People usually wait until the school or the doctor makes them get vaccinated,” Lyon said. “In many cases, it seems like they didn’t have to do it so they chose not to do it.” Enforcing school vaccination requirements appears to have helped encourage more vaccinations, Lyon said.

“There is also a strong movement by the anti-vaccination community to get into this community,” Zucker said. Some groups have complained that they were targeted for years by an anti-vaccine group called PEACH, or Parents Teaching and Advocating for Children’s Health — including with pamphlets and robo-calls.

“It is amazing how much resources and funding they have,” Zucker said.

Fighting back takes time.

Measles is extremely infectious. An unvaccinated person has a 90 percent chance of becoming ill if exposed to the virus and the virus itself is unusually transmissible. It can hang in the air and infect people even hours after an infectious person has left a room.

Puder said parents are often vague about their fears. “Parents will say things ‘like so many vaccines’ or ‘my child is too small to get so many vaccines’,” he said. “When the fear is not put into words ... it’s hard to respond.”

Many of the cases are among people who have skipped regular medical visits, Puder said. “I know in our group, it’s the kids you don’t see that don’t get vaccinated,” he said. "It's the group that is interested in alternative medicine."

And some pediatricians have enabled the vaccine-shy. “There are a few pediatricians who are more tolerant of vaccine-refusing parents,” he said. “I know who they are and I am not going to say their names.”

Although some people consider measles a benign childhood illness, it can kill. Measles causes encephalitis and pneumonia and before mass vaccination began in the 1980s, measles killed nearly 2.6 million people a year, according to the World Health Organization. It still kills more than 100,000 people a year, mostly children under five.

So every time a case is identified, public health workers must track down everyone else that patient was in contact with over the preceding days and weeks and check to make sure they do not become infected. “Whenever you get a confirmed case of measles, you have to interview the person and find out everywhere they have been and every person they met during the communicable period,” Lyon said.

But efforts are slowly working.

“It’s a clear and present danger right here in our community."

“I think we are on top of the outbreaks. The issue is to get all these kids vaccinated and to get a very strong campaign to raise public awareness,” Zucker said.

According to the CDC, 2018 was not worst recent year on record for measles in the U.S. As of Dec. 1, 292 cases were reported nationwide. In 2014, 667 cases were reported, fueled in large part by an outbreak traced to travelers returning from the Philippines and linked to Disneyland, as well as unvaccinated Amish communities in Ohio.

And anti-vaccine propagandists helped set the stage for an outbreak of measles among Somali immigrants in Minnesota in 2017.

This story was updated to reflect three new cases of measles as of Jan. 10.