One of the most important aspects of the fight to eradicate polio is detecting where the disease is present. This continuous surveillance is complicated and costly. Ninety percent of people infected with the virus show no symptoms, and those who do usually have mild symptoms such as fever, fatigue, and headaches. Only one in every 200 cases of the illness results in paralysis, which means that for every child with signs of paralysis, several hundred are carrying the disease and may not show it.

Vigilance is key to successful surveillance, says Michel Zaffran, director of polio eradication at WHO. “We need to go and investigate a case of paralysis, take specimens, and analyze it. This level of vigilance needs to continue in all of the places that no longer have polio to make sure we are really without polio. This is a hidden cost to the program that people don’t realize is absolutely necessary to maintain.”