Thanks to an effective vaccine, the polio virus has disappeared from most of the world. But, a few cases remain in areas that are among the most difficult to reach.

To eradicate the virus completely, medical teams are trying to vaccinate children in three countries. The wild polio virus still exists in Afghanistan, Pakistan and Nigeria. But unrest, conflict and the extreme remoteness of these areas has kept thousands of children from receiving polio vaccines this year.

Andrew Etsano works as an immunization expert for the Red Cross. He says that one of the difficulties the group faces is how often people in these areas move. They often flee economic problems, political situations or violence.

Etsano says that, as a result, people take the virus from one place to another. And medical teams cannot always track the migrants in order to vaccinate the children.

Dr. John Vertefeuille is with the U.S. government's health protection agency, the Centers for Disease Control and Prevention (CDC). He says the CDC uses technology to track migrant groups. Researchers look at images from satellites for clues. Structures that have been repaired and roads without grass growing on them may show researchers that people are living nearby.

Local is better

But when it comes to actually vaccinating people, experts note that local teams are best. Such teams are likely to be aware of an area's culture and customs.

For example, in many places women vaccinate people because they can go into homes, talk to other women and gain access to the children.

In other places, soldiers vaccinate children when they take over an area run by anti-government forces. But these vaccination teams have to move quickly during short breaks in the fighting. They also have to deliver several doses of the vaccine in a short period of time.

Even when teams are not providing vaccinations, community volunteers can warn health officials of a polio outbreak. Some volunteers have mobile phones to report if any child becomes unable to use their arms or legs.

The goal is to vaccinate everyone before the virus has a chance to change, making the existing vaccines less effective.

How long do we have to do this?

Efforts to end polio completely have been largely effective.

Since 1988, when the Global Polio Eradication Program began, the number of polio cases has dropped by 99.9 percent. In 2017, only 22 people were reported to be crippled by the disease, says the World Health Organization.

But the program's success has introduced new problems. Financial supporters, leaders, health workers and the public are tired of dealing with the problem, especially those who have never seen polio.

John Vertefeuille said, "The last mile is a complicated mile."

But, he and other experts warn, if polio continues to exist anywhere, it could once again spread.

I'm Jonathan Evans.

Kelly Jean Kelly adapted this story for VOA Learning English from a report by the Associated Press. Mario Ritter was the editor.

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eradicate –v. to completely remove

remoteness –n. the quality of being far away from other things

immunization –n. the process of giving someone a vaccine to prevent infection

track –v. to follow and observe

dose –n. an amount of a medicine taken at one time

crippled –adj. to be unable to move or walk normally