In Washington State, 71 people have had the measles since the start of 2019. An ongoing outbreak in Rockland County, New York, has spread too 143 people. So far this year, there have been 206 cases of the disease in the United States.

But measles is still technically eliminated in the United States. The disease was declared eliminated, after an effective vaccination program, in 2000—a public health designation which means a certain ailment has stopped actively spreading within one geographic area. However, eliminated doesn’t suggest the infectious agent is gone forever.

“Even if you eliminate the disease in one area, it’s still around in other areas, so there’s always the threat it can be imported,” says James Goodson, epidemiologist and senior measles scientist at the Centers for Disease Control and Prevention (CDC). “Mostly, you see U.S. travelers who go abroad and bring it back.” And if they come into contact with people who haven’t been vaccinated, clusters of illness can appear.

For a disease to be gone entirely, it has to be eradicated, with zero cases reported anywhere in the world—a feat only achieved once, with smallpox. “Elimination is local,” says Walt Orenstein, associate director of the Emory Vaccine Center. “Eradication is global.”

Various elimination efforts are underway in countries around the world: In the United States, for example, diseases like yellow fever and polio have been eliminated, and the CDC is working to eliminate tuberculosis. The World Health Organization (WHO) hopes to eliminate malaria in at least 10 countries by 2020, and each WHO region has goals set to eliminate various diseases. Improved human papillomavirus (HPV) vaccination and cervical cancer screening could eliminate cervical cancer—infection with HPV is a main cause—in dozens of countries, according to new research published this month in The Lancet Oncology.

Elimination remains a major public health challenge. It requires sustained attention and effort to bring the number of cases of a disease down low enough, and keeping that number down is another battle. The transition from local regions of elimination to a global, overall eradication compounds those challenges, Orenstein says. “Eradication is a very unforgiving goal. One infection is too many. If I say I reduced a disease by 99.999 percent, you’d think it was wonderful. But for eradication, it’s a failure.”

On top of this, not every disease is even a candidate for eradication, Orenstein says. There are biological and technical criteria: a disease has to depend on humans to survive, and it can’t be able to live on its own in animals or soil—so tetanus, which is caused by bacteria that can live in soil, can’t be eradicated. There must also be practical ways to identify and diagnose cases of the disease, and there have to be effective ways to stop it from spreading, like a vaccines.

There are also economic and political considerations, which are just as difficult to navigate as the scientific problems. “Global eradication takes a commitment from all countries, some of which might be suffering from economic insecurity, or war. Those are hurdles that can get in the way,” Goodson says.

Smallpox was successfully eradicated because it fit all the requirements for an eradicable disease: It’s only transmitted person-to-person, the distinctive rash makes diagnosis easy, and the vaccine against it worked incredibly well.

“In addition, there was a great understanding of the disease burden itself,” Goodson says. “Smallpox was horrific. There was lot of death.” The political circumstances were beneficial, as well. At the end of World War II, a number of global organizations formed, which created the opportunity for a global infrastructure under the WHO, he says.

After the eradication of smallpox, efforts turned to polio. The number of cases worldwide has dropped by 99 percent since the late 1980s, and 80 percent of the world’s population, including the United States, now live in regions free of the disease. In 2018, there were only 33 cases reported worldwide. .

But getting the number of cases from a handful to zero comes with its own set of challenges, says Stephen Blount, director of Special Health Projects at the Carter Center, a human rights and public health nonprofit organization started by former president Jimmy Carter.

When the number of cases gets this small, he says, political issues become increasingly important. “It takes more time and energy and effort to find the last one, two, or 20 cases than [to] find them when there are hundreds.” People who make decisions around resource allocation might also see getting rid of the last handful of cases as a low priority, as there are always other conditions having greater impact.

However, Blount says, the last remnants of a disease won’t go away on their own. “It’ll almost certainly start to get bigger if you discontinue the human effort to drive cases down.”

Once a disease is eradicated, there’s no longer a need for a public health infrastructure to try and beat back the progress of a virus: If it’s not around, there’s no need for anyone to be vaccinated. But until the number of disease cases hits zero, researchers must invest similar effort and resources to keep the condition from spreading.

Measles is a prime example of the importance of constant management. Even though the United States’ successful elimination means that the disease is extremely uncommon, it’s important to remember that elimination is not eradication. Right now, maintaining high vaccination rates is the only way to prevent it from creeping back. “If you turn your attention away to another problem, the old problem will just come back,” Blount says.

The America’s eliminated measles in 2016, but the virus has since reemerged, especially in Venezuela, where the state vaccination programs have been interrupted after healthcare infrastructure broke down. Because the virus has been circulating continuously for more than 12 months, by definition, it’s no longer eliminated.

“If you take action for a long period of time, and get numbers down, but stop the intervention, it almost certainly will come back,” Blount says.

After polio is eradicated—which likely isn’t far off, Goodson says—measles is likely the next candidate for focused eradication efforts. It might be an easier task than eradicating polio, he says, because cases are easier to identify and the vaccine is more effective. However, notes Orenstein, the measles is highly infectious, much more so than polio, which adds an additional hurdle.

Ramping up a global measles eradication will also take an enormous amount of funding, Goodson says, “The biggest challenge for measles has been the identification of a large donor who would be willing to take on a large effort,” he says. For polio, that was the international service organization Rotary International stepped forward with polio.

It’s important to keep pushing towards eradication of diseases—even if they’ve been eliminated in large swaths of the world—to ensure the everyone benefits from public health efforts.

“The incidence of disease that lingers longest is going to impacting the most vulnerable communities,” he says. “Achieving the eradication goals is achieving health equity. It creates more of a disparity the longer you wait.”