There have been 1,250 measles cases in the U.S. this year.

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Some health experts warn measles cases could spike this summer as people travel abroad. Getty Images

Editor’s note: This is a developing story that’s been updated since it was first published. Healthline will continue to update this article when there’s new information.

Pockets of unvaccinated individuals across the United States have made it possible for measles to make a startling and unexpected comeback this year.

And now health officials in California are reporting that 1 person visited Disneyland while infected with the measles.

The last time a person with measles visited Disneyland in 2014, it caused a major outbreak of measles in the state with 131 people infected.

Now to combat the potential of a new outbreak, the County of Los Angeles Public Health Department is advising anyone who was at Disneyland on October 16, between 7:50 a.m. to 10:00 a.m, to review their immunization records and meet with a healthcare provider if they’re not immunized or if they have a compromised immune system. This could mean pregnant women, older adults, or children under age 5.

“For those who are not protected, measles is a highly contagious and potentially severe disease that initially causes fever, cough, red, watery eyes, and, finally, a rash,” said Dr. Muntu Davis, MPH, Los Angeles County Health Officer in a statement.

“Measles is spread by air and by direct contact even before you know have it,” he continued. “The MMR immunization is a very effective measure to protect yourself and to prevent the unintentional spread of this potentially serious infection to others.”

There have been 19 cases of measles in Los Angeles this year. But across the country, measles cases have risen dramatically this year.

This latest case in California is just another headache for public health officials, who have been battling a record number of measles infections this year.

As of October 3, the Centers for Disease Control and Prevention (CDC) reported there have been 1,250 cases of measles so far this year.

This is the most cases of measles the country has seen since 1992, which was 2,126 cases.

“Also, despite outbreaks, people continue to choose to not immunize their children against medical advice,” Dr. Purvi Parikh, a clinical assistant professor of pediatric infectious disease with New York University Langone Health, told Healthline in an earlier interview.

Public health officials are working hard to increase vaccination rates in order to keep measles from becoming endemic in the United States again.

In addition, New York eliminated religious exemptions for vaccinations earlier this year.

The state is now among 5 others — California, Arizona, West Virginia, Mississippi, and Maine — that don’t allow religious exemptions.

Earlier this year the CDC issued a warning to travelers going to Europe, where over tens of thousands of people have had measles this year.

At least 90 people have died from measles in Europe this year.

“Measles is the most highly contagious virus that we know of, and there are still substantial numbers of children in affected populations who are not being vaccinated, so they remain susceptible. Over time, measles will find many of these children,” Dr. William Schaffner, the medical director of the National Foundation for Infectious Diseases (NFID) and a professor of infectious diseases at Vanderbilt University School of Medicine said in an earlier interview.

Many people associate measles with a rash, but the virus can be very serious, even life threatening — especially for infants and children.

About 1 in 5 unvaccinated people who get measles will be hospitalized. And nearly 1 in 20 children who contract the measles virus will develop pneumonia, which is the most common cause of death in young children, according to the CDC.

“People don’t realize that measles is more than a simple fever and rash illness. There can be severe long-term disability from measles infection,” said Dr. Christelle Ilboudo, an infectious disease expert with University of Missouri Health Care.

With pockets of unvaccinated people, the measles virus was close to becoming endemic or constantly present in the U.S. again this year.

However, the CDC reported this month that the U.S. is currently still holding onto its 20-year measles elimination status. This means that despite the rise in cases this year, there wasn’t endemic transmission of the virus for a full year.

If we ever lost elimination status, it could have a huge global impact and diminish other countries’ efforts to eliminate measles, Schaffner predicted.

“Upon seeing the U.S. example, other countries may decide it’s not that important to eliminate measles, and they may redirect resources away from it, which would be very unfortunate,” Schaffner said.

The disease has been eliminated from the United States since 2000, and the only reason we’re seeing it again is because groups of people have refused to get vaccinated, experts say.

“As long as there are pockets of susceptible people who contact each other, there is the potential for developing sustained transmission and becoming endemic,” Dr. Walter A. Orenstein, a professor and associate director with the Emory Vaccine Center and a previous president of the NFID, explained in an earlier interview.

Herd immunity — or the concept that a virus can’t spread when about 95 percent of a population is immune — has been the key to keeping certain diseases, like measles, out of the United States.

However, when people stop getting vaccinated, a community can lose herd immunity and diseases can return.

“An immunization is the one healthcare decision that affects other people as much as you due to herd immunity,” Parikh explained. “By vaccinating one person, you are protecting multiple and thus reducing spread of the disease.”

The measles-mumps-rubella (MMR) vaccine is incredibly effective and has already been proven to eradicate the disease in a very safe way.

CDC and public health officials have been working to break down vaccination myths, ensuring people have access to vaccines, and conducting thorough investigations of each outbreak to identify the sources.

Certain communities, like New York, have declared public health emergencies and ordered mandatory vaccinations in an effort to get more people vaccinated.

While these orders have been controversial among anti-vaccine and religious freedom activists, public health officials hope it will protect public health and prevent further transmissions.

“Public health officials have an additional challenge because their responsibility is to reach out to these communities of like-minded folks who are withholding vaccination,” Schaffner said.

There’s a vast amount of people in the United States who have never seen measles, and may be especially vulnerable to misinformation, he added.

Rather than take a punitive approach, Schaffner recommends public health officials listen to people’s concerns and mindfully address their vaccine hesitancy.

“People needs facts,” he said, “but also reassurance and comfort.”