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(Bloomberg) – Polio is spreading to countries previously considered free of the crippling disease, and that’s a global health emergency, according to the World Health Organization (WHO).

Pakistan, Cameroon and Syria pose the greatest risk of exporting the virus to other countries, and they should ensure that residents have been vaccinated before they travel, WHO said in a statement after a meeting of its emergency committee.

This is only the second time WHO has declared a public health emergency of international concern. The first international public health emergency declaration was a response to the 2009 influenza pandemic.

The virus that causes polio was once driven to the brink of extinction. Today, military conflicts are giving the virus a chance to make a comeback.

The number of cases reached a record low of 223 globally in 2012 and jumped to 417 last year, according to WHO. There have been 74 cases this year, including 59 in Pakistan, during what is usually polio’s low season, the WHO said.

The disease’s spread, if unchecked, “could result in failure to eradicate globally one of the world’s most serious, vaccine-preventable diseases,” Bruce Aylward, the WHO’s assistant director general for polio, emergencies and country collaboration, told reporters in Geneva today. “The consequences of further international spread are particularly acute today given the large number of polio-free but conflict- torn and fragile states which have severely compromised routine immunization services.”

While the number of cases in Afghanistan and Nigeria dropped by more than half last year, figures jumped 60 percent in Pakistan, where vaccination efforts have been hampered by rumors the shots cause infertility, and after the U.S. Central Intelligence Agency used a fake vaccination program to help hunt down Osama bin Laden. Twenty polio vaccinators and nine police officers assigned to guard them were killed in Pakistan last year, according to Rotary.

“Conflict makes it very difficult for the vaccinators to get to the children who need vaccine,” David Heymann, a professor of infectious diseases at the London School of Hygiene and Tropical Medicine, said in an interview before the WHO’s announcement. “It’s been more difficult to finish than had been hoped.”

The polio virus, which is spread through feces, attacks the nervous system and can cause paralysis within hours, and death in as many as 10 percent of its victims. There is no cure. The disease can be prevented by vaccines made by companies including Sanofi and GlaxoSmithKline P.L.C.

Cases of polio, which paralyzed large numbers of people around the globe for generations and crippled the late U.S. President Franklin D. Roosevelt, have dropped 99 percent since 1988, largely thanks to the global vaccination campaign backed by Bill and Melinda Gates.

The resurgence of the virus “reminds us that, until it’s eradicated, it’s going to spread internationally and it’s going to find and paralyze susceptible kids,” Aylward said.

A big outbreak of polio could be a concern even in countries with successful vaccination programs, because as many as 1 percent of children under the age of 6 who have received only three polio vaccine doses may be susceptible to a polio infection, and about 10 percent of children ages 18 months and younger – who typically have received only two polio vaccine doses – may be vulnerable, according to data from the U.S. Centers for Disease Control and Prevention.

Allison Bell contributed information to this report.

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