The World Health Organization declared the recent spread of polio to be a health emergency of international concern on Monday, following the virus' recent transmission from Pakistan, Cameroon, and Syria to other states in Africa and the Middle East after being pushed to the edge of eradication several years ago. The WHO recommended that these three countries see that all of their residents and long-term visitors do not leave the country without receiving the polio vaccine. Countries where the virus has been seen but has not spread from — Afghanistan, Equatorial Guinea, Ethiopia, Iraq, Israel, Somalia, and Nigeria — are recommended to encourage those leaving the country to also receive the vaccine.

"The current situation stands in stark contrast to the near-cessation of international spread."Though polio has been wiped out across the developed world, international aid organizations have had a far harder time replicating this in every country, in part due to regional and local conflicts. "The current situation stands in stark contrast to the near-cessation of international spread of wild poliovirus from January 2012 through the 2013 low transmission season for this disease," the WHO writes. "If unchecked, this situation could result in failure to eradicate globally one of the world’s most serious vaccine preventable diseases."

Polio's high transmission season began this month and will continue through June, and the WHO says that it will take a "coordinated international response" in order to prevent further spread of wild poliovirus, which can still infect people who have received the inactivated vaccine. The WHO notes that the spread of polio is a particularly significant threat because of the large number of "conflict-torn and fragile" countries, which may not have the infrastructure to deal with the virus should it return. The WHO will reassess the situation three months from now, at which point it may adjust its recommendations.