A measles outbreak in Rockland County, New York, has forced officials to declare a state of emergency in the sixth and largest outbreak across the United States this year, according to the CDC.

The New York County has identified 157 measles cases since October, causing health officials to ban anyone under 18 and not vaccinated against measles from public places. The ban will last for 30 days or until the entire community is vaccinated.

Public places includes shopping centers, restaurants, schools and places of worship. Outdoor gathering places, such as parks and public playgrounds, are not included.

Other measles outbreaks – defined as three or more cases – have hit California, Illinois, Texas, Washington and New York City. Clark County, Washington, had the second-largest outbreak of the year, with 73 documented cases of measles.

The CDC has confirmed more than 300 individual cases of measles in 15 states this year as of March 21. For all of 2018, there were 372 cases.

Here's what you should know about the measles and this outbreak:

What is measles? What are the symptoms?

Measles is an extremely contagious illness caused by a virus that is spread through the air.

People infected develop a red spotted rash that starts inside the mouth and spreads all over the body. Symptoms include fevers as high as 104 degrees, a cough, runny nose and red, watery eyes.

How serious is it?

In 2017, about 110,000 died of measles, mostly among children younger than 5, around the world, according to the World Health Organization. Young children and adults older than 20 are more likely to suffer measles complications that can be deadly.

As many as one out of every 20 children infected with measles gets pneumonia, the most common cause of death from measles in young children, according to health officials. One in every 1,000 children with measles develops swelling of the brain that can cause deafness or an intellectual disability. Pregnant women with measles might give birth early or have a low-weight baby.

Is there a cure?

There is no specific treatment available for measles.

How contagious is it?

Measles is so contagious that 90 percent of unvaccinated people who come in contact with an infected person will get the virus, according to the Centers for Disease Control and Prevention. The virus can linger in the air for up to two hours after an infected person coughs or sneezes. The virus can spread four days before and after symptoms appear.

More:Son defies mom, chooses to get vaccinated at 18: 'God knows how I'm still alive'

More:NY county bans kids from public places who are unvaccinated against measles

How effective is the vaccine?

The measles two-dose vaccine is 97 percent effective against the virus, according to the CDC.

How many people have been affected?

Rockland County Public Health officials have identified 157 confirmed cases.

Children younger than 1 year old make up 14.7 percent of the confirmed cases. Children ages 1 to 3 years old make up 26.3 percent of the cases, children 4 to 6 make up 16 percent and those 7 to 18 years old are 28.2 percent. Adults 19 years or older are 14.7 percent of the cases.

Of the 157 cases, 82.7 percent have not had measles, mumps and rubella vaccines, 9.6 percent had an unknown status and nearly 8 percent had either one or two MMR vaccines.

Where are people being infected?

The outbreak announcement comes on the heels of county health officials announcing six new exposure sites, including shops, a supermarket, a bus loop, a local Target and others. These are the first outbreak sites authorities have warned about since Thanksgiving of last year.

Why aren't people vaccinating?

People choosing not to vaccinate have become a global health threat in 2019, the World Health Organization has reported. The CDC recognized that the number of children who aren't being vaccinated by 24 months old has been gradually increasing.

Some parents opt not to vaccinate because of the discredited belief that vaccines are linked to autism. The CDC has said that there is no link and that there are no ingredients in vaccines that could cause autism.

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