International bodies which strive to finish off the dreaded polio from the face of the earth, at least by the coming years, are increasingly turning skeptical facing a number of challenges.

Caused by the poliomyelitis virus, the disease is known for withering limbs. The virus attacks the brain and spinal cord, causing paralysis or death.

Considering the development of polio vaccine as a boon and miracle of the 20th century medicine, public health officials and volunteers around the world had committed themselves to eliminating the disease from the planet by the year 2000.

About two billion kids have been vaccinated so far. The World Health Organization estimates that incidence of the disease has been cut down more than 99 percent and saving some five million from paralysis or death.

But now after six years past the deadline and $4 billion spent, many experts fear that total polio eradication is far from assured. There were 1,936 cases in 2005. And the virus still lurks in certain pockets of the world. This year in addition to India and Nigeria, cases have been reported in Somalia, Niger, Afghanistan, Bangladesh and Indonesia.

Within a couple of years, 18 once polio-free countries have had outbreaks traceable to Nigeria. Though most have since been tamed, Indonesia and Nigeria itself remain major worries. In 2001, there were fewer than 500 confirmed cases of polio paralysis in the world. Last year, the number jumped to more than 1,900 — and each paralyzed child means another 200 "silent carriers" spreading the disease.

Experts fear that the ambitious drive against polio is embroiled in a number of obstacles which are cultural, logistical, resources and simple exhaustion. Interest lags as the number of cases falls. Fatigue sets in among volunteers, donors and average people. Yet even one unvaccinated child can allow a new pocket of the disease to bloom.

While Nigeria struggles to restart its campaign, India, where the prospect of conquering polio is more intimidating that it uses more than half the world's two billion polio vaccine doses each year, has long made an extraordinary commitment to wipe out polio.

No eradication effort against any disease has been as well financed or as comprehensive as the polio drive. The world has donated billions of dollars for polio eradication. Japan and Great Britain have given more than $250 million, and Canada, the Netherlands, the European Commission and the World Bank each have given more than $100 million. Far and away the biggest donors have been the United States and Rotary International, which initiated the "gift to the 21st century" idea. Each has given more than $500 million.

Despite all the efforts the polio campaign has dragged on. Many even think that the official caseload figures on www.polioeradication.org are incomplete and that the World Health Organization may not actually know every pocket of virus in the world.

Even if it does, and even if all the world's polio cases can be wiped out, problems that are now being nearly ignored in the all-out effort to corral the last few cases will suddenly loom large, they suspect. For example, as a precaution, vaccination must be continued for many years after the last case is found, polio experts agree. (Nearly every American child is still immunized — albeit with a killed vaccine given by injection — even though polio was virtually wiped out in the United States in the 1960's.)

But in about one in three million doses, the live oral vaccine used in poor countries can mutate back into a wild-type virus that can infect and paralyze victims. They are used, however, because they give better immunity than the killed vaccine and are easier to administer, studies suggest.