ON THE MARCH TO ERADICATE CHILD ILLNESS

Dr. Bruce Aylward is yielding no ground. As coordinator of the World Health Organization's $4 billion Global Polio Eradication Initiative, Aylward runs a worldwide immunization program that is supposed to eliminate the virus forever by the end of this year. He's still not ready to push back the schedule, even though cases of the devastating childhood illness have been popping up in countries like Indonesia and Yemen, where it was wiped out long ago. "The virus has never been in this much trouble," he insists. When the global campaign began in 1988, the disease was paralyzing

350,000 or more victims a year on five continents--mostly children. So far this year, the number of confirmed cases hasn't passed the low hundreds. Now, says Aylward, the essential thing is to finish wiping out the disease. "If we blink," he says, "it will be not hundreds, but hundreds of thousands."

People forget too quickly the horrors of sicknesses like polio. In 1954, the year before Jonas Salk introduced his vaccine, the virus killed more than 3,000 Americans and crippled roughly 20,000 others. Today the United States has been effectively polio- free for more than a quarter century. That success story is hardly unique. A century ago, before childhood immunization became routine in America, the upper-respiratory infection known as diphtheria was a worse killer than cancer. Now the United States gets an average of three cases a year. Whooping cough, measles, mumps and rubella have become rarities in America. Wider vaccine coverage could further reduce the burden of other important illnesses already in decline, like hepatitis A, hepatitis B, chickenpox and invasive disease from Haemophilus influenzae type b (Hib for short), a bacterial meningitis that can cause deafness or mental retardation in children when it doesn't kill them outright.

But the economics of vaccines can be as cruel as any disease. A third of the world gets none of the childhood immunizations that are routine in the West. More than 2 million children a year die as a result, according to Dr. Julian Lob-Levyt, executive secretary of the Global Alliance for Vaccines and Immunization. For drugmakers, the profitability of developing and distributing vaccines tends to be marginal at best, and medical aid to the Third World is chronically underfunded. And yet for humanity as a whole, immunization programs pay huge dividends, not only in lives saved but in reduced medical and social burdens. "The cost benefit of vaccines is overwhelmingly the best investment yield in biomedicine and health," says Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "It is much more economical to prevent a disease than to treat a disease." "[Immunization] is the most cost-effective technology in public health, bar any," says Lob-Levyt.

That fact is a powerful motivation for scientists, relief groups, philanthropists and others in the fight against communicable diseases. Earlier this year the Bill & Melinda Gates Foundation teamed up with the Norwegian government to make a joint pledge of $1 billion for standard childhood immunizations in the Third World. Meanwhile, immunologists are working to expand their arsenal. A few of the most promising targets:

Dengue Health workers have spent decades in a losing fight to stop the mosquito that transmits dengue. Every year the hot-climate viral illness infects as many as 100 million people, many of them children. Roughly 500,000 end up hospitalized for intravenous feedings, oxygen and sometimes transfusions. Up to 5 percent die anyway. "Parents go through hell," says Scott Halstead, research director for the Pediatric Dengue Vaccine Initiative. One form of the disease, dengue hemorrhagic fever, mimics the Ebola virus: victims bleed from their bodily orifices as they die. Now scientists at Aventis and GlaxoSmithKline are closing in on vaccines to ward off the disease. Studies are underway in Vietnam, Thailand and Nicaragua, and Halstead says the result could be available by 2010--for under $2 a dose, he hopes. The best thing about dengue is that it needs human hosts to survive. If a population is immunized, the disease will be gone, no matter how bad the mosquitoes get.