Alarmed by the spread of polio to several fragile countries, the World Health Organization declared a global health emergency Monday for only the second time since regulations permitting it to do so were adopted in 2007.
Just two years ago -- after a quarter-century-long campaign that vaccinated billions of children -- the paralyzing virus was near eradication; now, health officials say that goal could evaporate if swift action is not taken.
Pakistan, Syria and Cameroon have recently allowed the virus to spread -- to Afghanistan, Iraq and Equatorial Guinea, respectively -- and should take extraordinary measures to stop it, the WHO said.
"Things are going in the wrong direction and have to get back on track before something terrible happens," WHO spokesman Gregory Hartl said. "So we're saying to the Pakistanis, the Syrians and the Cameroonians, 'You've really got to get your acts together.' "
The declaration, which effectively imposes travel restrictions on the three nations, represented a newly aggressive stance by the WHO, which has often bent to pressure from member states demanding no consequences, even as epidemics raged inside their borders and sometimes slipped over them.
"This is a fundamental shift in the program," said Bruce Aylward, the WHO's chief of polio eradication, who is a Canadian physician and epidemiologist. "This is the countries of the world signaling that they will no longer tolerate the spread of the virus from the countries that aren't finished."
The emergency was declared though the total number of known cases this year is still relatively small: 68 as of April 30, compared with 24 by that date last year. What most alarmed experts, Mr. Hartl said, was that the virus was on the move during what is normally the low transmission season from January to April.
"What we don't want is cases moving into places like the Central African Republic, South Sudan or the Ukraine," said Rebecca Martin, director of global immunization for the federal Centers for Disease Control and Prevention in Atlanta, which has provided money and expertise to the eradication campaign since it began in 1988.
Fighting the virus normally includes several rounds of vaccination for all young children in a target country. But, in an unusual step, the agency also said all residents of Pakistan, Syria and Cameroon, of all ages, should be vaccinated before traveling abroad, and that this restriction should be retained until one year after the last "exported case."
It also said another seven countries should "encourage" all their would-be travelers to get vaccinated. Those are Afghanistan, Equatorial Guinea, Ethiopia, Iraq, Israel, Nigeria and Somalia. Israel has had no confirmed human cases of the disease, but a Pakistan strain of the virus has been detected in sewage in Tel Aviv and elsewhere.
While the WHO has no enforcement power, the regulations are part of a 2007 global health treaty saying all signatories "should ensure" that WHO-recommended steps are taken. That applies to Pakistan, Syria and Cameroon. The other seven only need to "encourage" those steps.
But nations could use the document to refuse to admit migrants, visitors or even business travelers who lack vaccination cards.
Polio, short for poliomyletis, is a highly contagious virus spread in feces; although only one case in 200 causes symptoms, the hardest-hit victims can be paralyzed or killed. With so many silent carriers, even one confirmed case is considered a serious outbreak. There is no cure.
Unlike influenza or other winter viruses, polio thrives in hot weather. Cases start rising in the summer and often explode when the monsoon rains break the summer heat, flooding sewage-choked gutters and bathing the feet of romping children with virus, which they pick up by touching their feet or a ball and then putting a finger in a mouth.
Though the disease primarily strikes children, evidence has mounted that it also crosses borders in adults, such as traders, smugglers and migrant workers.
With 54 of this year's 68 new infections, Pakistan is by far the riskiest country, Dr. Aylward said. Polio has never been eliminated there, Taliban factions have forbidden vaccinations in North Waziristan for years, and those elsewhere have murdered vaccine teams.
Syria has had only one confirmed case of polio this year, but it had 13 cases last October, the first there since 1999. Before the uprising began in 2011, Syria had a 90 percent vaccination rate, but it fell rapidly in war-torn areas.
Cameroon's outbreak is of a strain from Nigeria, which previously had more cases than any nation in the world but which has had only two so far this year.