It is, says the World Health Organization, "an extraordinary event." Polio is spreading to a degree that constitutes a public health emergency.

The global drive to wipe out the virus had driven the number of polio cases down from 300,000 in the late 1980s to just 417 cases last year. The World Health Organization has set a goal of wiping out polio by 2018.

But this year, polio has been reported in 10 countries, and there are fears the number could rise. Bruce Aylward, the head of WHO's polio program, says if the international spread isn't halted, the virus could easily re-establish itself, particularly in conflict-torn countries like the Central African Republic and South Sudan. The unrest makes it difficult to sustain vaccination efforts, and poor sanitary conditions cause the disease to spread.

Although polio mainly afflicts children under 6, a WHO emergency committee has stated that adults are to blame. The committee noted that there is "increasing evidence that adult travelers [from Pakistan, Syria and Cameroon] contributed" to the polio surge.

As a result, the World Health Organization has taken the unusual step of ordering these three countries to vaccinate any resident who travels internationally. In addition, WHO is calling for the three countries to continue efforts to inoculate their children. The mandate was issued by the director-general's Emergency Committee on International Health Regulations.

Aylward says this focus on travelers is critical to stem the virus, which causes paralysis and can be fatal.

"The Pakistan virus has been reported from Israel, from Iraq, from Syria, and was also found at one point in the sewage in the West Bank and Gaza," Aylward says.

Polio virus that was genetically linked to Pakistan also turned up in Cairo last year and caused an outbreak in China in 2011.

The new WHO rules require Pakistan, Syria and Cameroon to vaccinate all international travelers at least four weeks prior to departure. Travelers from these countries will have to carry proof of vaccination.

This program could prove difficult to implement in Syria, given the chaos of the civil war and the almost 9 million Syrians displaced from their homes. But Aylward is optimistic.

"I believe it's very much possible," he says, "if not four to 12 weeks prior to travel — because a lot of travel there right now is, as everyone knows, quite unpredictable and on very short notice — certainly at point of departure."

India instituted similar requirements for travelers in March and added one more: Visitors from a country with ongoing polio cases must be vaccinated six weeks prior to entering India.

Naveen Thacker, who has worked on India's polio eradication effort for more than two decades, says the new WHO rules do more than just stop an individual traveler from unknowingly spreading the virus stowed away in his gut.

"It's a very welcome step because it builds a sense of urgency in these countries," he says. "I remember India was one of the last countries [with smallpox]. And our then-prime minister, Mrs. Indira Gandhi, just telling the nation, this is a matter of our pride, and then the whole nation followed."

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