Polio, the world's great crippler of children, will survive into the next century. But if all goes well, it will also be the new millennium's first great casualty.

Over the next 12 months, an international effort will attempt to drive polio into the most exclusive category of human disease--those eradicated from Earth. The list has one item: smallpox.

The campaign against this disease, launched in 1988 and coordinated by the World Health Organization, is the largest public health endeavor ever undertaken.

Over the last decade, the campaign has temporarily stopped wars so that whole populations of children could be immunized simultaneously. Logistical feats, such as the vaccination of 134 million Indian children on one day in 1998, have no match in history. Counting part-time volunteers, about 10 million people worked in the campaign this year.

Because polio cases must be sought out and distinguished from other forms of paralysis, the campaign has required establishment of disease surveillance systems in some of the poorest and most chaotic nations on Earth. In all, the effort will cost about $2 billion. In inflation-adjusted dollars, this is about one-third more than the cost of the campaign to eradicate smallpox, which lasted from 1967 to 1977. More than half the money will be spent during the "accelerated phase" now beginning, and in the three years of intensive surveillance that will follow the last case.

Although the Americas have been free of the disease since 1991, no child here--or anywhere else--can safely forgo polio vaccination as long as the virus exists anywhere on Earth.

So far this year, 1,240 confirmed cases of polio have been found worldwide, although the actual number may be two or three times that. In 1988, there were 350,000. Although disappearing fast, the disease still exists in 23 countries and is suspected to be circulating in six others, where conditions have hampered a thorough search.

The goal of stopping transmission of the virus by the end of 2000 will be "tough," said Harry F. Hull, a pediatrician and epidemiologist who is the senior adviser to the program. "Not impossibly tough, but tough."

Among the many obstacles to the final push toward eradication is the supply of oral polio vaccine. About 2.5 billion doses will be used in the campaign next year, about twice as much as has ever been used in a single year. Combined with the 300 million doses or so that will be used for "routine" polio vaccination, that is all there is in the world.

"The manufacturers are running now at maximum capacity. There's probably enough, but it's very tight," said Hans Everts, the campaign's chief of vaccine supply.

Although its most serious effects are on the nervous system, poliovirus is actually an "enterovirus"--a microbe that replicates in the intestinal tract. It is passed in feces for weeks after infection, and also in airborne droplets. Highly contagious, the virus moves explosively through non-immune populations, especially where hygiene is poor. The vast majority of cases occur in children, in whom it has historically been among the leading causes of disability.

Like a few other microbes, polio has the necessary characteristics for "eradicability."

The virus has no reservoir in the natural world. It doesn't live in soil (like the bacterium that causes tetanus) and it doesn't infect animals (like the virus that causes influenza.) Halt transmission in human beings, and polio dies out.

Equally necessary is a safe and effective method of preventing the infection, which came in the form of the Salk injected vaccine, introduced in 1955, and Sabin oral form, two years later. (In the United States, the vaccines drove the number of cases of paralytic polio caused by "wild" virus from about 16,000 per year in the early 1950s to zero by 1980.) Vaccination causes prolonged immunity, so people don't become reinfected. Further, polio has no chronic "carriers"--people who survive infection but continue to transmit the microbe.

Still, polio has one trait that makes eradication unusually difficult: Only one in 200 cases causes weakness or paralysis, the cardinal feature distinguishing it from other intestinal bugs. Most polio infections go undetected. This is unlike smallpox, where nearly every case gave rise to a dramatic rash.

When epidemiologists find a newly paralyzed child, it's imperative they determine whether the cause is polio. (There are numerous non-polio causes of muscle weakness in childhood, with the autoimmune disorder known as Guillain-Barre syndrome the most common.) If it is polio, investigators can be certain many dozens of undetected cases lurk nearby.

Because of this, the eradication campaign has required more than vaccination. It also has required that countries create permanent systems of disease surveillance, which is not easy in poor, rural countries with little medical infrastructure.

Under WHO guidelines, everyone from doctors at urban hospitals to paramedic-like workers at village "health posts" must report new cases of muscle weakness in children. Often, government health workers also actively look for such cases.

Once found, two samples of feces must be collected from the sick child within 14 days and sent to one of about 100 WHO-certified laboratories, where they will be cultured for polio virus. Before a country is certified polio-free, it must show it is identifying at least one case of non-polio paralysis for every 100,000 children under age 15--evidence the system is working and finding no polio.

Without question, however, the backbone of the eradication campaign is vaccination on an unprecedented scale.

Two drops of a liquid containing live, but weakened, virus are squeezed into the mouth. The usual schedule is three doses, given in infancy and early childhood. Once swallowed, the virus multiplies and stimulates immunity. The vaccine-derived virus can be transmitted to others, immunizing them too.

"I like to joke that the only education necessary to administer polio vaccine is the ability to count to two," Hull said in a recent lecture at the Johns Hopkins University School of Hygiene and Public Health. "This has allowed us to use millions of non-medical volunteers."

Oral polio vaccine is part of the WHO-recommended childhood vaccines used worldwide. For most of the decade, however, the eradication campaign has employed a strategy of supplementing routine vaccination with massive events called "national immunization days" (NIDs). Officials choose a day--usually in the cool, dry season, when polio is in low prevalence--in which all children under the age of 5 in a country are given polio vaccine, regardless of whether they have been previously vaccinated.

"Running an NID is like running an election," said Bruce Aylward, 37, a Canadian physician and long-time WHO field worker, who two years ago was chosen to lead the final stage of the eradication campaign.

In many ways, the logistical obstacles to such events are advantageous, Aylward said, because they require everything from national governments to village councils to buy into the effort. And because NIDs are done in pairs one month apart, at least once a year, the commitment is lasting.

In December 1996 and January 1997, NIDs were held in Bangladesh, Burma, China, India, Nepal, Pakistan and Thailand. In all, 243 million children were vaccinated--about 38 percent of the world's children under age 5. On Dec. 7, 1997, 127 million children were vaccinated in India. Five weeks later, 134 million were vaccinated.

Cease-fires have been brokered in Sudan, Sierra Leone, Angola and several other countries so NIDs could be held. In his lecture at Johns Hopkins, Hull showed a photograph of the president of Sierra Leone seated on a couch next to his chief adversary in a bloody civil war. The two leaders and an aide to each wore "Kick Polio Out of Africa" T-shirts.

India, which had about half the world's confirmed cases this year, will hold four nationwide NIDs next year, and two more in the eight northern states where polio is most prevalent. India will use about 1 billion doses of vaccine.

NIDs, however, are insufficient to eradicate polio. Nearly everywhere, they have to be supplemented with "mopping-up" campaigns.

The most dramatic example occurred in Cambodia. Nine cases of polio appeared in the Mekong River area in early 1997, despite three years of NIDs in which more than 90 percent of children were reached. An epidemiologist noticed all nine children lived on boats.

He mentioned this to Cambodian officials who, Hull said, were incredulous such a population existed. The epidemiologist hired an airplane and photographed thousands of boats, with families on board, along the waterway and its tributaries. About 1 million Cambodian and Vietnamese children were subsequently vaccinated in campaigns that included boat-to-boat visits. There have been no new cases since March 1997.

Oral polio vaccine is not without risk. It causes paralytic polio in about one in every 1 million children. During NIDs, there is no attempt to explain the risks and benefits of vaccination to every parent.

As in the smallpox eradication campaign, informed consent of a formal nature comes from the national or regional health ministry. Often, however, details such as the existence of vaccine-associated polio are discussed in media coverage preceding mass immunizations, Aylward said. No child is forced to be immunized.

The polio campaign is being funded by many sources, governmental and private. The largest private donor is Rotary International, a service organization of business and professional men and women. that has about 29,000 clubs in 162 countries. It has raised more than $300 million. Recently, the Bill and Melinda Gates Foundation contributed $50 million.