The rapid spread of polio in the conflict zones of Pakistan, Syria, and Cameroon has prompted the World Health Organization to declare an international health emergency. An emergency committee said on Monday that the three countries have allowed the virus to spread, reversing decades of effort made toward bring the disease to near-complete eradication. The WHO urged the countries to contain the virus by vaccinating or re-vaccinating children, and said that adult travelers should carry government-approved documents showing proof of vaccination. At the end of 2013, 60 percent of polio cases were the result of the international spread of the virus, according to the WHO.

The re-emergence of polio has also seeped into neighboring countries; the virus has spread into Afghanistan from Pakistan, into Iraq from Syria, and into Equatorial Guinea from Cameroon, the committee said. As Rick Gladstone reports in The New York Times, adult travelers are partly to blame for the spread, as well as unvaccinated communities. Polio largely affects children under 6, but ongoing conflict means health workers have found difficulty in reaching communities to vaccinate. In Pakistan’s tribal areas, in particular, health workers are often targets of violence as Taliban fighters often accuse them of being Western spies, or the programs themselves of being part of CIA plots (which they sometimes are.) 

Afghanistan, Ethiopia, Israel and Nigeria also have contained infections, but there is an ongoing risk that the disease could spread from those countries, too.