PARIS — Alarmed by the spread of polio from conflict zones in three continents, the World Health Organization declared an international health emergency on Monday in an effort to contain the paralyzing virus, which officials thought two years ago had been nearly eradicated.
An emergency committee convened by the organization announced in Geneva that three countries — Pakistan, Syria and Cameroon — had allowed the virus to spread, and should take extraordinary measures to combat it, including making sure that all children in those countries are inoculated or reinoculated. Dr. Bruce Aylward, the W.H.O. official in charge of polio eradication, said during a telephone news conference that citizens of those countries who travel abroad should be vaccinated before they leave and should carry an internationally recognized certificate as proof.
Though the disease primarily strikes children under 6, the committee said there was “increasing evidence that adult travelers contributed” to the recent spread of polio from Pakistan to Afghanistan, from Syria to Iraq, and from Cameroon to Equatorial Guinea during what health officials said was the low season for polio transmission, between January and April.
Ten countries are now affected by the new wild polio virus, Dr. Aylward said, including those six as well as Ethiopia, Israel, Nigeria and Somalia. In the case of Israel, he said that there were no confirmed human cases of the disease, but that the virus, which originated in Pakistan, had been detected in the country’s sewage.
According to the health organization, there were 417 known new cases of polio around the world in 2013, compared with 223 in 2012, the lowest on record. Three-fifths of the new cases in 2013 were in regions that had previously been free of polio, a consequence of conflict and the interruption of vaccination campaigns, the organization said.
“It can become endemic in the entire world if we do not complete the eradication of this disease,” Dr. Aylward said.
Polio, short for poliomyelitis, is a highly contagious viral infection that can lead to partial and sometimes fatal paralysis. It is preventable through immunization, but there is no cure. Because some people can carry the virus without displaying symptoms, and can unwittingly infect hundreds of other people, the World Health Organization considers even a single symptomatic polio case to constitute an epidemic.
Despite a concerted inoculation effort in the Middle East, global health officials warned in April that polio had spread to Iraq for the first time in 14 years, as refugees fleeing war-torn Syria strained Iraq’s already fragile health care system.
Dr. Aylward said Pakistan was a notable exporter of the virus. Eradication efforts there have been hampered by the hostility of local leaders in the conservative tribal areas of the country and the conflict with Taliban militants, who have repeatedly attacked health workers administering vaccines. The violence grew worse after American forces located and killed Osama bin Laden in 2011 using information gathered in part by a doctor under the guise of a vaccination campaign. The doctor was later convicted of treason.
Dr. Aylward said Pakistan had made some progress by improving security in vulnerable areas, and had installed vaccination booths at land borders with Afghanistan, China, and Iran. But it still has not begun vaccination in some parts of the country. The number of cases recorded in Pakistan rose to 93 last year from 58 in 2012.
Pakistani officials said on Monday that they would step up efforts to fight the spread of the disease. Saira Afzal Tarar, the state minister of health, was quoted by Pakistani news outlets as saying that the government would announce its strategy on Wednesday after meetings with provincial officials. The World Health Organization and other agencies will be asked to provide more vaccine, she said, and special counters may be set up at airports.
“The best way is that there should be vaccination at the airport and a card issued before departure,” Ms. Tarar said.
Some countries have made major gains in combating the disease. As recently as 2009, India had more new cases of polio than any other country, but after an immunization program, within three years it was detecting no new cases.