The happiest place on Earth isn’t so cheerful these days. In December, a large number of people started coming down with measles after visiting Disneyland in California. About 80 measles cases have been reported in the state, an unusually large number for a virus that has long been under control.

In 2000, the measles virus reached elimination status in the United States. Elimination is not the same as eradication, which indicates that a disease has been extinguished worldwide. Elimination means a disease is no longer native to a country, but cases could still exist because of infected people traveling to that country. Despite this definition, the recent outbreak doesn’t look good for sustaining elimination, as more and more unvaccinated people contract the disease.

More than 600 cases of measles were reported across the country in 2014, the highest count in 20 years. Experts say the reason for the uptick in cases is partly because more parents have actively chosen not to vaccinate their children. The Centers for Disease Control and Eradication recommends children get two doses of the vaccine, one when they’re 12 to 15 months old and the other when they’re 4 to 6 years old.

Vaccination is the only known way to prevent the disease. The more people who are immune, the less the disease can spread, which creates what is known as herd immunity. This means that even if there is a small number of people who are not vaccinated, they’ll generally be protected from a large outbreak, as long as a large percentage of people are vaccinated.

The Federal Drug Administration licensed the first measles vaccination in 1963, contributing to a steady decline in reported cases. Here’s an excerpt from a February 1962 Popular Science article title “Not-so-funny measles” discussing two potential measles vaccines in development. At that point, there was an estimated 4 million annual cases of measles in the United States.

But this time, instead of being “greeted with smiling relief,” the multi-state outbreak has been faced with a CDC health advisory and increased attention. Medical scientists are still not amused.