FANISAU, Nigeria - If it were possible to wind back the centuries, Halima Umar's village would probably look much as it does today. Umar and her neighbors fetch water by lowering a bucket into a hand-dug well, toil in fields of millet and guinea corn, and sleep in houses made of mud, leaves and animal hair, the walls sagging like sandcastles struck by an ocean wave.

In January last year, Nigerian health workers knocked on Umar's door, offering her newborn daughter a free dose of polio vaccine.

Two drops of the oral polio vaccine taken at least three times as a child, the health workers told her, would protect her daughter, Zaliha, from the crippling virus for life.

But Umar turned the vaccinators away. Most of her neighbors did the same, some hiding children under their beds.

They had heard a rumor circulating through the hot, dusty villages of northern Nigeria that the vaccine had been contaminated with an anti-fertility agent that would sterilize their children or perhaps infect them with the AIDS virus, all part of an American plot to depopulate the developing world. The villagers believed it.

It didn't matter that the World Health Organization, the Nigerian government and dozens of doctors and scientists dismissed the allegations as baseless, assuring the public that the vaccine taken by 1 billion children worldwide is safe.

"We just didn't like the vaccine," says Umar.

During the following three months, vaccinators returned to Umar's neighborhood two more times to immunize the children. On both occasions, the shy 22-year-old mother politely but firmly declined their offer.

Then in June, 10-month-old Zaliha developed a fever and diarrhea. Zaliha, who had recently learned to crawl, suddenly could no longer sit up. A few days later, paralysis, polio's damning trademark, arrived, leaving her legs limp and lifeless.

Aided by rumor, fear and misinformation, polio is making a comeback in Nigeria, reinfecting polio-free regions and moving into West and Central Africa, putting 15 million children at risk and threatening the goal of a polio-free world.

The outbreak comes just as polio appeared to be backed into a corner. In 1988, when the WHO launched a campaign to eradicate the disease, there were more than 350,000 known cases in 125 countries. Today there are about 500 cases in just seven countries: Nigeria, India, Pakistan, Egypt, Afghanistan, Niger and Somalia.

Six of these countries are on track to eliminate polio by the WHO's deadline of the end of 2004. But Nigeria, with nearly half of the cases, threatens to derail the worldwide effort.

Most of the country's 286 cases have occurred in Nigeria's predominantly Muslim northern states, where anti-Western feelings intensified by the U.S.-led wars in Afghanistan and Iraq make the population suspicious of any program supported by the United States.

Radical Muslim clerics preached the dangers of the vaccine during Friday prayers. Villagers chased, threatened and assaulted vaccinators. Frustrated, some vaccination teams dumped thousands of doses of the vaccine rather than face angry villagers, WHO officials say.

In response to the outcry, three northern Nigerian states, including Umar's home state of Kano, canceled immunization campaigns last year, allowing the virus to spread and get a foothold in neighboring countries.

Nearly a dozen children were crippled last year in Burkina Faso, Ghana, Niger, Togo and Chad from a poliovirus genetically linked to northern Nigeria.

"Polio is now spreading to areas that have been polio-free for years," says David Heymann, an epidemiologist based in Geneva who is leading the World Health Organization's polio eradication campaign. "All it takes is for someone to hop on an airplane ... to start reinfecting another part of the world."

Such fears have left WHO and local health officials scrambling to contain the spread of the virus and dispel the rumors so the vaccinations can begin again.

But if there is one measure of the challenges public health officials face here, it is Umar's defiance. Standing in her doorway on a recent morning, holding her crippled daughter, she expresses no regret.