The 16-year effort to eradicate polio from the globe is threatening to unravel in Africa, with the report yesterday that a 10th country -- Sudan -- now harbors the disease after being free of it for three years.

Nigeria, where the current upsurge began, now has five times as many cases of polio as it did this time last year, officials at the World Health Organization in Geneva said. New cases have been found in 30 of its 37 states, with most in the Muslim north, where immunization efforts virtually stopped last year because of political rivalries and rumors about the vaccine's safety.

Africa's season of high transmission for the virus is about to begin, raising the specter that thousands of children may contract the disease in the year it was supposed to be extirpated.

"Right when it should have been disappearing during the low season, this virus was gathering steam to come roaring out," R. Bruce Aylward, global coordinator of the eradication campaign at WHO, said in a telephone briefing.

Last winter, health ministers of the countries where polio remains endemic won international agreement to step up the deadline for eradication to the end of this year, a goal that may now be in jeopardy. David L. Heymann, another WHO leader of the eradication effort, said, however, that despite the setbacks, he and others are not ready to abandon that goal.

"I think there's still a chance. Africa did it before -- except in Nigeria and Niger -- and there's no reason they can't do it again," he said.

Smallpox is the only human disease ever to be eradicated. The original target date for polio eradication was pushed back several years ago from 2000 to 2005. Worldwide, there have been 333 cases this year, compared with 183 at this time last year.

Polio's appearance in Sudan and continuing spread in Nigeria has lent urgency to a request last month by a group of African health ministers that WHO help them run a coordinated 22-country immunization campaign this fall. The campaigns will aim to vaccinate every child younger than 5 -- about 74 million -- during a one-week period in late October, and again a month later. The vaccine is given as two drops of liquid in the mouth.

The fall campaign will cost about $100 million, Heymann said. About $25 million will be needed by early August.

"That's a good question," he said of where the money will come from. "We are working hard to get it."

About $3.1 billion has been spent on polio eradication since the initiative was launched in 1988 at the suggestion of the service club Rotary International, which has raised about $600 million.

Immunization rates generally need to be more than 80 percent to break the person-to-person chain of virus transmission. They were briefly that high several years ago in the African countries that were polio-free until the recent outbreak. In some cases, however, the rates have now dropped to 50 percent or lower, WHO officials reported.

Only about 1 in every 200 polio cases results in the limb weakness or paralysis that allow physicians and nurses to recognize the disease. Consequently, finding a dozen cases implies that at least a thousand people are harboring, and possibly transmitting, the virus.

The disease's appearance in Sudan, a country full of refugees living in temporary camps, was especially worrisome because people in crowded environments with poor sanitation are at greatest risk. Polio transmission follows a "fecal-oral" route. The virus grows in the intestine and is discharged in feces. It is then ingested by uninfected people, often through contaminated water.

Until a year ago, the only sub-Saharan countries where polio was found were Nigeria and Niger. Since then, virus traceable to northern Nigeria has caused cases in Burkina-Faso, Chad, Benin, Ivory Coast, Togo, Cameroon, Ghana, Central African Republic and Botswana.

Three hundred cases of polio have been found in the region this year, compared with 58 last year. Nigeria accounts for 257 cases, including 60 reported this week.

Countries with renewed infections are not considered "endemic" for polio until numerous cases appear over a period of at least six months. Besides Nigeria and Niger, the only other polio-endemic countries are Egypt, Afghanistan, Pakistan and India.

The Nigerian outbreak is centered in Kano, a state with about 3 million children younger than 5 that last vaccinated widely in March 2003. Along with several nearby states, Kano stopped vaccinating because of rumors that the vaccine was part of a U.S. or Western plot against Muslims and would cause infertility or AIDS. Some local leaders also used suspicions about the vaccine as a vehicle to resist the country's Christian-dominated national government.

Recently, a delegation of 22 people from northern Nigeria went to Indonesia, where the vaccine was tested and a manufacturing plant was inspected, and were convinced the vaccine was not contaminated.

Heymann said he spoke yesterday to Kano's governor, who assured him there is no longer any official resistance to resuming vaccination. Kano will use vaccine made in Indonesia, a Muslim country, in future campaigns.

A public education and advertising campaign will begin soon, and Kano-wide polio vaccination may resume in July or August, he said.

Plans are underway to vaccinate in 22 countries this fall. Children get two drops of liquid in the mouth.