Driven by ongoing outbreaks in New York state, the number of cases of measles in 2019 has surpassed 700, the highest level in the United States since the disease was declared eliminated in 2000.

According to the latest figures from the Centers for Disease Control and Prevention, 704 people have been diagnosed with measles so far this year. The majority of these cases occurred in children under the age of 18 who had not been vaccinated.

The news comes as lawmakers in New York announced they are introducing legislation that would remove all non-medical exemptions from vaccine requirements for children in the state.

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"Immediacy of action is critical," Rockland County Executive Ed Day said in a press conference on Monday. There have been 202 confirmed cases of measles in the suburban county north of New York City.

In another ongoing outbreak, Brooklyn and Queens have seen 390 cases of measles since October. Most of these cases, as well as those in Rockland County, have involved members of the ultra-Orthodox Jewish community where misinformation on the safety of vaccines has spread.

"Measles is not a harmless childhood illness"

Measles is extremely contagious and can be serious for all age groups, but it is most dangerous for children under 5 and adults over 70.

According to the CDC, as many as one out of every 20 children with measles develops pneumonia, the most common cause of death from measles in young kids.

"Measles is not a harmless childhood illness but a dangerous highly contagious disease," Health and Human Services Secretary Alex Azar said in a CDC press briefing on Monday.

About one out of every 1,000 children with measles will develop swelling of the brain, or encephalitis, which can lead to convulsions and leave the child deaf or with an intellectual disability.

For every 1,000 children who get the disease, the CDC estimates one or two will die from it. Measles can also cause pregnant women to give birth prematurely or have a low-birth-weight baby.

Of the cases that have occurred so far in 2019, 9 percent of patients have been hospitalized and 3 percent have come down with pneumonia, health officials said. No deaths have been reported.

How does measles spread?

Measles is spread through the air when an infected person coughs or sneezes. It is so contagious that if one person has it, up to 90 percent of those close to that person who are not immune will also become infected.

"The problem is you're infectious even before you have obvious symptoms like the rash," said CBS News chief medical correspondent Dr. Jon LaPook. "You won't even know and can infect other people."

The disease was declared eliminated in the U.S. in 2000 thanks to the MMR (measles, mumps, rubella) vaccine, which meant there were no longer sustained outbreaks. But when an international traveler gets exposed to measles overseas brings the disease into the country, growing pockets of unvaccinated communities within the U.S. enable the disease to spread.

While vaccines are required for school children across the country, 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. The proposed legislation in New York would eliminate the option in that state. California passed a similar law tightening its requirements in 2015 after a measles outbreak that spread at Disneyland.

Vaccinations are safe

Health officials say current measles outbreaks in the U.S. are fueled by the anti-vaccination movement, which questions the safety of vaccines and frequently spreads false information on social media. Yet experts emphasize that vaccines are safe and the best tool available to prevent measles.

The mistaken belief took off in 1998, when a doctor in the U.K. published a now discredited study claiming the MMR vaccine was linked to autism. "That has subsequently been totally discredited," LaPook said. "The lead author lost his license."

But the investigation took over a decade and the claim spread fear among parents, leading to a small but vocal faction that makes up the current anti-vax movement.

Experts say overall vaccination rates of 90 to 95 percent are needed to provide "herd immunity," which helps keep outbreaks at bay and protect babies who are too young to be vaccinated and others who can't get the vaccine for medical reasons.

"Vaccine preventable diseases belong in the history books, not in our emergency rooms," Azar said. "The suffering we are seeing today is completely avoidable."

Who needs to be vaccinated?

The CDC recommends all children get two doses of the MMR vaccine, starting with the first dose at 12 to 15 months of age, and the second dose between the ages of 4 and 6 years old. It says one dose of the MMR vaccine is 93 percent effective at preventing measles, and two doses are about 97 percent effective.

Health officials are also recommend infants 6 through 11 months receive one dose of the vaccine before international travel.

Teenagers or adults who have not had measles or who have not been vaccinated should get two doses of the MMR vaccine at least 28 days apart.

Anyone who does not know their vaccine status should speak to their health care provider.