Last summer, Nigeria celebrated having gone a year without a case of polio. But then last month, just before meeting the two-year mark, two children in Nigeria were diagnosed with polio paralysis, and a third case has now been detected. All three cases are in Borno state (in northeastern Nigeria) in areas liberated from Boko Haram militants. NPR’s Jason Beaubien reports:

Dr. Chima Ohuabunwo, an epidemiologist who has been working with the U.S. Centers for Disease Control and Prevention in Nigeria for the past five years, says Boko Haram has cut off parts of Borno state, in Nigeria’s northeast, from the rest of the world.

“There’s been no direct in and out movement of persons, or access to health care, for the past two to three years,” Ohuabunwo says.

Earlier this year, he says, half of Borno state was a no-go zone. Government health care workers and international relief groups, including polio vaccination teams, could be attacked or killed if they tried to enter those areas. At the same time, Boko Haram was pillaging farms and destroying health clinics.

“Of about 38 secondary health care facilities in the entire state, 16 were totally burnt down by these insurgents,” Ohuabunwo says.

Now, Beaubiean reports, Nigeria is conducting immunization campaigns in accessible parts of Borno state, and administering vaccinations from posts on roads just outside of the Boko Haram-controlled areas. The World Health Organization is planning additional vaccination campaigns in Cameroon, Chad, and Niger, which border northeastern Nigeria. For National Geographic, Maryn McKenna explains what health officials have learned from these two cases about the virus’s likely circulation:

The WHO said genetic analysis of the virus from the two children and their families, who live in different parts of Borno, reveals that it is most like a strain collected in Borno in 2011. That implies polio has never been eliminated from the region but instead was circulating silently, even though Nigeria had not detected any polio cases anywhere in the country since July 2014. The brutal math of polio epidemiology—experts consider that only one in every 200 cases is detected—makes it likely that polio could be present throughout the area.

Last year, when Nigeria appeared to be polio-free, it looked like the disease could be eradicated from Africa — and, before too long, the world. Cases still occur in Afghanistan and in Pakistan, where gunmen have killed more than 100 members of inoculation teams since December 2012. These attacks followed the revelation that the CIA had used a vaccination campaign as a ruse in its hunt for Osama Bin Laden.

Vaccines have prevented thousands of children from suffering polio’s crippling and sometimes deadly effects. Unless we can eradicate the disease, though, children in areas already ravaged by terrorism and other harms will remain at risk.