Why is polio so hard to beat?

Story highlights

  • In August, Nigeria reported four cases of polio
  • Health officials believe more cases may have gone undetected

(CNN)For just more than two years, Nigerian health officials celebrated a country free of the paralyzing effects of the poliovirus.

The achievement was yet another step toward eradicating polio for good, both in the country and globally, with only two countries -- Afghanistan and Pakistan -- remaining endemic for the disease.
But in August, celebrations were put on hold with the announcement of four new cases of polio in Nigeria. First, the World Health Organization reported on August 11 that two children had been paralyzed by the poliovirus, and two more reported cases followed in subsequent weeks in Nigeria's northeastern state of Borno. This state is where the militant group Boko Haram is based and has challenged immunization efforts.
    Before August, no cases had been reported in the country -- or on the continent -- since July 2014. Just one more year without polio would have meant the WHO African region could be declared polio-free.
    "We are deeply saddened by the news," Dr. Matshidiso Moeti, the WHO's regional director for Africa, said in a statement at the time.