NEWS BRIEF Measles, one of the world’s most infectious diseases, was once everywhere. Two decades ago, North, Central, and South America committed themselves to rid the region of the disease. The Pan American Health Organization (PAHO) on Tuesday said the Americas had accomplished this goal, making it the world’s first region without endemic cases of measles.

The news came during a meeting of World Health Organization ministers from throughout the Americas. It makes the fifth vaccine-preventable disease the region has eliminated, including smallpox and rubella. Measles, a particularly deadly disease, can cause brain swelling, blindness, and pneumonia.

It’s important to note, however, that measles has not disappeared from the Americas. It’s just not considered endemic. Here’s how PAHO explains what this means for the Americas:

Measles transmission had been considered interrupted in the Region since 2002, when the last endemic case was reported in the Americas. However, as the disease had continued to circulate in other parts the world, some countries in the Americas experienced imported cases. The International Expert Committee reviewed evidence on measles elimination presented by all the countries of the Region between 2015 and August 2016 and decided that it met the established criteria for elimination. The process included six years of work with countries to document evidence of the elimination.

Before researchers developed a vaccine in 1980, measles killed 2.6 million people in the world each year. That has since come down to about 150,000. As my colleague Adrienne LaFrance wrote, the nature of measles makes it hard to get rid of. The virus remains contagious in the air for up to two hours, and someone can spread measles four days before they see a rash. The United Kingdom has already wiped it out once, only to see it return.

PAHO’s strategy leaned heavily on giving children the MMR (measles-mumps-rubella) vaccination, then following up every four years. This means areas without regular vaccinations are especially vulnerable, like some communities along the California coast where parents don’t vaccinate their children. Nearly two years ago, an unvaccinated child at Disneyland in California contracted measles and spread it to more than 70 children around the area before it was contained. The Centers for Disease Control and Prevention lists 54 current measles cases in the U.S. right now, but all these have been contracted from foreign countries.