In a fractious world, there is one ­enemy that has had a unique ability to compel people from all countries to lock arms: poliovirus. For more than 15 years, the World Health Organization (WHO) has coördinated mass- immunization campaigns in an effort to eradicate poliovirus, a goal it hopes attain by the end of this year. If this Global Polio Eradication Initiative succeeds, it will join the smallpox eradication program as one of the greatest medical triumphs in history. So there was a great gasp in 2003 when northern Nigeria broke ranks with the rest of the world and banned the polio vaccine, triggering an outbreak that soon spread to 12 neighboring countries -- and illustrating once again how easily the virus can take advantage of any chink in our collective armor. Then again, the Nigerian setback may unintentionally have given the initiative the added fuel that it needs to cross the finish line on time.

Since its inception, the polio eradication program has reduced the incidence of paralytic polio by 99 percent, from some 350,000 cases a year to fewer than 1,000. As of 2003, the virus was circulating only in Nigeria and five other countries. But in the middle of that year, Muslim clerics in northern Nigeria denounced the vaccine, claiming it contained hormones intended to sterilize girls or that it was contaminated with HIV. By the fall, northern Nigerian politicians and health ministers had banned the immunization campaigns in Kano and two other states -- a move seen as a nod to the prestige of the Muslim clerics and a slap at both the West and the country's Christian president, Olusegun Obasanjo. By year-end, Nigeria had reported more than 355 polio cases, surpassing India and Pakistan for the first time. "Nigeria is a painful example of the potential impact of vaccine refusal," says Daniel Salmon of Johns Hopkins University's Bloomberg School of Public Health.