Measles, a disease that was considered eradicated in the United States nearly 20 years ago, has made a return, and could spread even further if nothing is done to slow down the current rate of infection.

Today there are outbreaks in more than a dozen states across the country and New York has reached a breaking point. On Tuesday, New York City Mayor Bill de Blasio declared a public health emergency and ordered mandatory vaccinations to halt an outbreak concentrated amongst ultra-Orthodox Jews in Williamsburg and Brooklyn.

This is possibly the broadest vaccination order in the United States in over three decades — but did it have to come to this? And will this mandate be enough, if an entire community is afraid of vaccines and refuses to budge?

According to medical ethicist Art Caplan, the disease is very likely to continue to spread if people continue to refuse vaccinations.

“Measles is, I think, the most contagious and infectious disease we have,” Caplan said in an interview with Boston Public Radio Wednesday. “It’s 18 times more contagious than HIV, for example. Really easy to get it, you can get it by being in a space that someone who had it was in and breathed into the air just hours later. It’s highly, highly contagious.”

According to Caplan, there is a misconception that measles is not dangerous, when it could have very serious impacts for those without strong immune systems.

“A lot of people think, 'Oh, I had the measles when I was a kid, and I was home from school for a little bit, it wasn’t that bad and it’s not that serious of a disease.' … It’s pretty serious,” Caplan said. “It can cause deafness, it can cause your brain to swell and give you intellectual impairment, and measles also can put you in the hospital."

In New York, five patients have been admitted to the intensive care unit because of measles exposure.

“If you have cancer treatment or transplant or some immune disease or a newborn, then you’re really at risk for the measles,” Caplan said.

“With the measles outbreak here like we’ve never had for a long time,” Caplan said, “there’s a real public health crisis here.”

Caplan is the Drs. William F and Virginia Connolly Mitty Chair, and director of the Division of Medical Ethics at NYU Langone Medical Center. He’s also the co-host of the Everyday Ethics podcast.