Asked if she would accept the vaccine if she could wind the clock back to that day health workers first knocked on her door, she does not hesitate with her answer.

"No," says Umar, shaking her head. "I would do the same."

Eradication program

Few people in the developed world think about poliomyelitis, but during the first half of the 20th century, it was one of the most feared diseases.

The highly infectious virus can strike anyone at any age, although children younger than 5 are most vulnerable. Thriving in unsanitary conditions, the virus enters the body through the mouth and multiplies in the intestine. If it invades the nervous system, it can lead to paralysis within five days.

With no known cure, polio left victims crippled for life, rolling in wheelchairs, hobbling on crutches or spending their lives in coffin-like "iron lungs," huge cylinders that helped the most severely stricken victims breathe.

In 1955 Dr. Jonas Salk discovered a polio vaccine. But it was Dr. Albert Sabin's development in 1961 of a live polio vaccine taken orally that made mass immunizations and the dream of a polio-free world possible. Simple to administer and costing pennies a dose, the oral polio vaccine could be distributed widely and inexpensively by volunteers with minimal training.

Led by the WHO, Rotary International, the U.S. Centers for Disease Control and Prevention and the United Nations Children's Fund, the anti-polio effort began in 1988 and has grown into one of the world's largest public health care initiatives, costing more than $2 billion so far.

More than 5 billion doses of the vaccine have been distributed door to door by 20 million volunteers and health workers traveling by train, helicopter, horseback, motorcycle, boat and on foot to deliver the doses to every child younger than 5.

Vaccinators persuaded soldiers battling in civil wars in Angola, Congo and Sudan to lay down their weapons for several days so that thousands of children could be protected from the disease. In parts of India, trains have not been allowed to leave their stations until all children under 5 were immunized.

Five million people who would otherwise be paralyzed are walking because they received the vaccine, according to the WHO.

But the campaign has dragged on longer than expected, missing its original deadline of 2000.

If there are further delays, health officials fear, the campaign could be undermined by donor fatigue and frustration.

If the campaign is successful, polio would be only the second disease ever to be eradicated. The first was smallpox in 1979.

Conspiracy theory

No one quite knows how the rumors about the vaccine's safety began, but local officials point to the northern state of Kano, which has the highest number of polio cases in Nigeria, as the likely source.

The state capital, Kano, developed as a center for camel caravans crossing the Sahara to trade slaves, ivory and kola nuts for salt, glass beads, weapons and cowrie shells.

Enveloped in a gray, exhaust-filled haze much of the day, the city of 1 million is dominated by the emir of its walled palace and the central mosque, where as many as 50,000 worshipers gather for Friday prayers.

The dominant tribe is the Hausa, whose culture is defined by Islamic faith. Boys study the Quran in outdoor schools under trees by the side of the road. Women are rarely seen outside the mud walls of their homes. Peace and order are maintained by the state's Sharia court system.

As in much of the Muslim world, anti-American passions run high among Kano's residents. In such a climate, rumors of plots involving the polio vaccine found a receptive audience.