Health officials in Rockland County, New York have declared a state of emergency amid an ongoing measles outbreak. The county is prohibiting unvaccinated children under the age of 18 from going out in public spaces for 30 days, CBS New York reports.

There have been 153 confirmed cases of measles in Rockland County, which sits about 40 miles north of New York City. The majority of cases have been in children, most of whom have not been vaccinated.

"We will not sit idly by while children in our community are at risk," County Executive Ed Day said in a press conference Tuesday. "This is a public health crisis and it's time we sound the alarm."

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The ban begins at midnight tonight and is in effect for 30 days. Public places affected by the order include include shopping centers, businesses, restaurants, schools, and places of worship. Anyone under 18 who hasn't been vaccinated will be banned from such places for 30 days or until the individual gets vaccinated. People who are medically unable to get vaccinated are exempt.

Day noted that this is the first effort of its kind nationally. "The circumstances we're facing here clearly calls for that," he said.

He emphasized that law enforcement will not be going around asking everyone for their vaccination records. However, parents of unvaccinated children "will be held accountable" if they are found to be in violation, Day said, and could face a class B misdemeanor charge.

Measles outbreak in Rockland County

In September of last year, an international traveler arrived in Rockland County with a suspected case of the measles. Six additional cases of measles from international travelers to the area followed, exposing more people to the virus. The outbreak was centered in an Orthodox Jewish community where many residents had not been vaccinated. Day noted that many Orthodox rabbis have been working with health officials to encourage vaccination in their community, so it's not a matter of religious objection.

The cases are currently clustered in eastern Ramapo, though officials warn exposure may occur anywhere in the county. The Health Department is working to limit exposures and offering free vaccine to boost the county's immunization rate.

Earlier this month, a federal judge barred 50 unvaccinated students from attending Rockland's Green Meadow Waldorf School for at least three weeks.

To prevent the spread of illness, officials are advising people who may have been exposed and have symptoms of measles to contact their health care provider, local clinic or emergency department before going in to seek medical care. This will help the facility take steps to ensure others won't be exposed to the virus.

Anyone told by a health care provider to "watch for measles" after possible exposure is advised to stay home and not go out in public, where they could expose others.

What are the symptoms of measles?

Measles is a highly contagious respiratory disease that is serious for small children but is easily preventable with a vaccine. Symptoms usually appear 10 to 12 days after exposure.

The first symptoms of measles typically include:

High fever

Cough

Runny nose

Red, watery eyes

Two or three days after the first symptoms, tiny white spots may appear inside the mouth. Then comes a rash that usually begins as flat red spots on the face at the hairline and spread downward to the neck, trunk, arms, legs, and feet.

Serious complications from measles can include pneumonia or encephalitis (brain swelling), leading to blindness or deafness, and in some cases death.

"Every new case is a roll of the dice" that could lead to life-threatening complications, Day said.

How does measles spread?

The measles virus is transmitted through the air when an infected person sneezes or coughs.

It is extremely contagious — much more so than Ebola, tuberculosis or influenza. The virus can live and be passed to someone for up to two hours after an infected person has left the room.

The disease is so contagious that 90 percent of people who are not immune will get sick if they're exposed to the virus.

The importance of the measles vaccine

One dose of the MMR vaccine — which protects against measles, mumps and rubella — is 93 percent effective, and two doses are about 97 percent effective.

However, once a person is infected, there is no specific treatment for measles, meaning vaccination is literally a life-saving tool.

Vaccines are required for school children, but almost all states allow exemptions for families who say it's against their religious beliefs, and 17 states allow a parent to opt out for philosophical or personal reasons.