Few causes merit greater celebration than the end of a disease. But despite the dedicated efforts of the last century, the world has only held such a celebration once—when smallpox was eradicated in 1977. Current generations who know smallpox only as a fading scar on the upper arm forget the impact that this global killer had over centuries. Its eradication in the United States alone has saved countless lives and at least $17 billion.

Today, the world is poised to add another disease to the list of those that will no longer threaten humans: polio. As difficult as smallpox eradication was, polio has presented an even tougher challenge. Some polio infections alert doctors with tell-tale paralysis, but for each of these cases, about 200 people may have only minor flu-like symptoms and can silently transmit the disease for weeks. As a logistical challenge, one observer has written, the difference between smallpox and polio eradication is “the difference between extinguishing a candle flame and putting out a forest fire.”

Yet we have never been closer to ending the disease. In 1988, there were an estimated 350,000 cases of polio worldwide. In 2005, the confirmed caseload has been slashed to just 760 people in 13 countries. Through national and international leadership, local heroism, and economic investments, immunization rates are climbing in most countries. In 2003, 415 million children in 55 countries were immunized during National Immunization Days, using more than 2.2 billion doses of oral polio vaccine. Most national health services have responded quickly to outbreaks. China, for example, stamped out a potential flare-up last year. The World Health Organization launched a massive preemptive vaccination campaign in Somalia to prevent an outbreak from spreading into the...