Just when it looked as if polio was headed toward eradication around the world, the disease is once again on the march.

The World Health Organization declared on Monday that the spread of the polio virus to new countries in 2014 had become “a public health emergency of international concern” that warranted aggressive measures to control transmission. It was timely advice on the eve of what is typically the onset of the high season for transmitting the virus.

Only two infectious diseases have ever been eradicated — smallpox and rinderpest, a viral cattle disease — but there were expectations that polio would soon join them. That hope dimmed this year when three countries where the polio virus was thought to be bottled up allowed the virus to be carried beyond their borders.

Pakistan, which has the largest number of domestic cases largely because Taliban factions have forbidden vaccinations in conservative tribal areas and attacked health care workers elsewhere, has spread the virus to neighboring Afghanistan. Syria, rived by civil conflict, has spread cases to neighboring Iraq, and Cameroon has spread cases to neighboring Equatorial Guinea. WHO also named seven other nations as infected with the polio virus but not yet exporting it: Afghanistan, Equatorial Guinea, Ethiopia, Iraq, Israel, Somalia and Nigeria. It said these nations should “encourage” their citizens to be vaccinated and obtain certificates of proof. And it urged all nations infected with polio to carry out more vigorous immunization campaigns.