In 2000, the United States declared that it had eliminated measles, a childhood infection that can be serious and deadly but is easily prevented with a vaccine. Now, according to the Centers for the Disease Control and Prevention (CDC), more than 700 cases of measles have been confirmed in the United States this year, making it the largest outbreak of the virus since its eradication, reports The New York Times.

The virus has been found in 22 states and has led to the hospitalization of 66 people. What’s more, the CDC discovered that more than 500 of the 704 people diagnosed with the measles weren’t vaccinated.

A majority of the cases were identified in mostly Orthodox Jewish communities in New York City and its suburbs. The outbreak has also spread to Detroit and Los Angeles, where almost 800 college students were ordered to stay home under quarantine until they could submit proof that they had been vaccinated. Several hundred remain under quarantine.

According to Robert Redfield, MD, director of the CDC, more than 94 percent of parents vaccinate their children against measles and other illnesses. But thousands of kids under 2 who haven’t been immunized are at risk of becoming sick during this outbreak.

Some parents don’t get their children vaccinated because they believe that vaccines are unsafe or may cause autism, a belief that’s been circulating for some time but has been debunked.

People who are allergic to parts of the vaccine or take treatments that suppress their immune system, such as cancer or organ-transplants meds, are also at risk of developing measles.

At this time, authorities haven’t announced any deaths from the surge in cases of the disease, but officials believe it’s possible. (On average, measles kills one of out every 1,000 victims even with modern health care.) Additionally, the longer the outbreak persists, the more likely the infection is to become a threat to public health once again.

However, the CDC says two doses of the measles vaccine, known as the MMR vaccine (for measles, mumps, rubella), is nearly 97 percent effective in preventing the virus.

The agency recommends that children receive the first dose of the vaccine between 12 and 15 months of age and the second dose at 4 through 6 years old. (During an outbreak, doctors may administer the shot to healthy children as young as 6 months old.)

Click here to read Real Health’s editor-in-chief’s thoughts on how many diseases could make a comeback if people stop getting vaccinated.