Polio is back in Nigeria, just when the World Health Organization thought the virus might be gone completely. Two kids have been paralyzed by the virus for the first time in two years, the WHO announced today. Both live in northeastern Nigeria, where it has been hard to vaccinate kids because the area is controlled by terrorist group Boko Haram. The government is now preparing an emergency immunization program as researchers fear a possible outbreak.

Many people might be infected and don’t know yet

Polio is one of the diseases we are closest to beating. Before today, the WHO thought polio was active only in Afghanistan and Pakistan. The new cases are both in Borno, where half a million kids haven’t been vaccinated because Boko Haram controls the area, according to Stephen Cochi, a senior polio program scientist at the Centers for Disease Control. Not only that, but the massacres and violence mean that people constantly enter and leave the region, making it hard to track disease.

The government hopes to vaccinate nearly 5 million kids in the northeast and could start the immunization program as early as next week. Though there are just two cases so far, others may be infected and not know it. Both cases were detected because the patients were paralyzed, but only 1 in 200 people with the virus have such extreme symptoms. This suggests that many people are infected and don’t know yet.

Nigeria has been a weak spot in vaccination efforts for years. In the past, religious leaders have pushed against vaccination, calling it part of a Western plan to sterilize Muslims.

Even by 2012, more than half of all worldwide polio cases were in Nigeria, which made the recent success so hopeful. For a region to be declared polio-free, WHO requires three years with no cases. It looked like Nigeria might be on the brink.

"This is a setback, but we need to double our effort to make sure we interrupt transmission," John F. Vertefeuille, director of polio eradication for the Centers for Disease Control and Prevention in Atlanta, told The New York Times.