In Yemen and Indonesia, international health workers are waging emergency immunization campaigns to stop new outbreaks there, and some experts fear the Nigerian flare-up, which has re-infected 16 formerly polio-free countries, could take hold in the Horn of Africa, where it could prove extremely difficult to oust again in troubled nations like Somalia.

"We're watching Yemen carefully," said Bruce Aylward, the World Health Organization's global polio eradication coordinator. "If the Horn of Africa gets all re-infected, that will be a tough one. If this spreads further through Ethiopia to Somalia, then we will have a real grind to get it out of those places."

Yemen officials Tuesday reported 41 new cases of polio across the country, and UNICEF said it is shipping 6 million doses of polio vaccine there in an effort to vaccinate every child under 5. Yemen, which until recently had not seen a polio case since 1996, has a current child vaccination rate of only about 50 percent, health officials said.

Parts of the country are remote and lawless, which may make the vaccination campaign difficult, health workers warned.

In northern Nigeria, however, where nearly 800 children were paralyzed by polio in 2004, the crippling virus is coming under control and has struck 54 children so far this year, according to WHO figures.

The turnaround in Nigeria is due largely to dramatic new support for immunization in the Muslim-dominated north, the product of a national and international campaign to convince regional leaders the polio vaccine is safe.

Governors in some northern Nigerian states began barring immunization efforts in mid-2003, after a few politically ambitious religious leaders charged that the polio vaccine contained contaminants that could sterilize children or infect them with HIV.

Because giving children full immunity to polio can take up to seven doses of vaccine administered at least a month apart, government immunization teams visited homes several times. And in a region rife with suspicion of the southern-led government and with relatively few cases of polio, "people began looking around and saying, `I don't see any disease. What's really going on here?''' Aylward said.

The problem worsened when initial testing of the vaccine found trace amounts of estrogen in a couple of samples. Scientists said the amounts were too small to have any effect, but vaccine opponents seized on the results as proof of a plot.

That view began to change in late 2003 and early 2004, as the number of polio cases in the region started to soar. To prove the safety of the vaccine, international health officials flew members of a newly formed polio commission -- including northern doubters -- to South Africa, Indonesia and India to look at the vaccine-manufacturing process and to conduct tests.

U.N. Secretary-General Kofi Annan sent a special ambassador to spur talk between northern governors and other Nigerian officials, and the governor of Kano state -- at the center of the epidemic and the controversy -- ordered new vaccine tests, which came up clean. By mid-2004, the Kano governor re-launched immunization efforts. Mothers rushed into the streets with their children to greet vaccination teams.

But by then, the disease -- which by 2003 had been contained to Nigeria, Niger, Egypt, India, Pakistan and Afghanistan -- had roared back across Africa. Cases began showing up across much of West and Central Africa, and as far south as Botswana. The disease hit the war-devastated Darfur region of Sudan and poor and densely populated Ethiopia.

The virus, which attacks the brain and spinal cord and can cause permanent paralysis in hours, is now considered re-established in six previously polio-free African countries -- Sudan, the Central African Republic, Ivory Coast, Chad, Burkina Faso and Mali.

Cases have also appeared in Saudi Arabia and, in recent weeks, Yemen and Indonesia. All show signs of a Nigerian origin.

That has forced WHO and other world health agencies to scramble in Asia and the Middle East to contain the new outbreaks -- 63 cases in Yemen, six in Indonesia -- before the disease spreads further. The risk is substantial, as routine vaccination for polio has faltered since the 1990s in many nations as cases of the disease -- and fear of it -- gradually vanished. In Indonesia, which had not seen a polio case since 1995, only about 75 to 80 percent of children are vaccinated, health officials said.

New epidemics are "a big concern," Aylward said. "We have to get this back under control."

One of the most frustrating aspects of the new outbreaks is that the mass vaccination campaigns they are prompting -- 100 million children targeted in 22 African countries, millions more in Yemen and Indonesia -- have diverted money and resources from the battle to eradicate polio from the nations where it remains endemic.

Vaccination rates among children in densely populated nations like Pakistan and India are growing, but at least 5 percent of children are still being missed, Aylward said. That means there is always the possibility of new outbreaks.

The reality, in Nigeria as in India, is "we can't miss a settlement, we can't miss a household, we can't miss a child," said Melissa Corkum, a WHO spokeswoman in Abuja.

"When there's virus circulating anywhere, any country is at risk," she said.

How successful the current vaccination push has been in Africa, Yemen and Indonesia will be evident only in August and September as the region heads into the polio high season. Then "if it's lurking somewhere, we're going to see it," Aylward said.

Even with the recent outbreaks, the number of polio cases worldwide -- 165 this year -- is still dramatically down from 1998, when worldwide eradication efforts began and the disease, which strikes mainly children under 5, was crippling an estimated 350,000 children a year in 125 countries.

But the successes are "very, very fragile," Aylward warned. "We were on the verge of eradicating this and we blinked. Now a half-million kids a year could be paralyzed by this disease again if we fail to exploit our opportunities to eradicate it. If we step back at this point, we're going to lose an opportunity that will be very difficult to explain to future generations."