A July 1 article incorrectly said that the campaign against polio is the second attempt in history to eradicate a human disease. If the campaign succeeds, polio will be the second human disease to have been eradicated, joining smallpox. But there have been unsuccessful attempts against several diseases, and an effort is underway to eradicate dracunculiasis, also known as Guinea worm disease. (Published 7/2/04)

The World Health Organization warned travelers to northern Nigeria yesterday to get polio vaccine boosters because of "uncontrolled transmission of polio virus" in the region.

The announcement comes as the region enters the season when polio transmission increases, typically rising five- to tenfold. There has been little or no polio immunization in Nigeria's northern states for more than a year because of rumors that the vaccine is a tool in a global plot to sterilize Muslims.

WHO's warning, announced in Geneva, is purely advisory. The organization has no power to force the immunization of travelers, or even to monitor it.

Nevertheless, the suggestion that visitors get a polio booster if they have not had one in four years further sounds the alarm that the outbreak in Nigeria presents an "extreme danger" and a "threat to every country," said David L. Heymann, a WHO physician helping lead the 16-year effort to eradicate polio.

Six countries in the world still have "endemic," or freely circulating, polio virus in their populations. All except Nigeria are in the late stages of eradication campaigns. That country has had 259 cases of paralysis from the disease this year -- nearly 80 percent of the world total. For every known infection that causes paralysis, health experts say, there are about 200 "silent," or undetected, ones.

Over the past 12 months, 10 countries in Africa that had been free of the disease for several years have recorded cases of polio traceable through genetic tests to microbes originating in northern Nigeria.

Both the epidemic and the resistance to immunization have centered in Kano state, but Heymann said yesterday that the government there has officially dropped its opposition. In a telephone news conference, he said Kano's governor recently told him that health workers have begun planning a vaccination campaign for later this month and another for August.

Kano will use vaccine manufactured in Indonesia, a Muslim country. Whether that, as well as assurances by government and religious leaders that the vaccine is safe, will be enough to bring about widespread participation in the campaign remains uncertain. Polio vaccination campaigns target all children younger than 5; Kano has 3.3 million of them.

WHO, Rotary International, UNICEF and the U.S. Centers for Disease Control and Prevention -- the four organizations running the eradication initiative -- announced last month that emergency vaccination campaigns will be held simultaneously in 22 countries in the fall to try to stop the outbreak. This will add about $100 million to the cost of the eradication effort, which to date has cost $3.1 billion.

Heymann said WHO will convene a panel of experts next week to review the pattern of the recent African cases and to consider whether any other recommendations about travel might help stop the outbreak.

The most recently infected country is Sudan, which recorded a case in Darfur, a region where the number of refugees is rising, along with reports of genocide against certain ethnic groups.

The campaign against polio is only the second attempt in history to eradicate a human disease. Smallpox was eliminated in the late 1970s.