More than three decades after smallpox was eradicated, an international struggle has re-emerged with new intensity about whether to destroy the only known specimens of the virus that causes one of the worst scourges to plague humanity.

Some public health authorities, infectious disease specialists and national security experts maintain that the time has come to finally autoclave hundreds of vials of the pathogen held in two high-security government labs in the United States and Russia.

"We feel the world would be safer without having these stocks in existence. Why risk it escaping and resurging again?" said Lim Li Ching, a researcher at the Third World Network, an international research and advocacy group based in Malaysia.

But the U.S. and Russian governments, which have repeatedly delayed incinerating the samples, are fighting for another stay of execution. Scientists need the living virus, they say, to make a better vaccine and finish developing the first treatments just in case the deadly microbe is somehow unleashed again - by accident, by a bioterrorist or by re-creating it from the computerized records of its DNA sequences.

"This is not research for the sake of research," said Ali Khan, director of the Office of Public Health Preparedness and Response at the federal Centers for Disease Control and Prevention (CDC) in Atlanta, which guards one of the collections containing about 450 strains. "We still have work to do to protect the public if, God forbid, the virus was ever released back into the population." The debate will culminate in May, when the World Health Assembly, which governs the United Nations' World Health Organization (WHO), will vote whether to condemn the virus to extinction. The WHO has no power to compel destruction of the stocks, but the United States and Russia would risk international condemnation by defying the body.

"It's not a black-and-white story. You have to balance a lot of different things," said Pierre Formenty, a WHO scientist in Geneva.

Smallpox is caused by the variola virus, an ancient pathogen that naturally infects humans and no other species. Victims suffer high fevers, wracking headaches and body pain, pus-filled skin ulcers, vomiting and bleeding. About one-third die. Many survivors are left blind and scarred. There is no approved treatment.

"For centuries, repeated epidemics swept across continents, decimating populations and changing the course of history," according to a WHO history.

By rapidly vaccinating everyone around each victim beginning in 1966, a WHO-sponsored campaign led by D.A. Henderson of Johns Hopkins finally pushed the virus back to a single last natural case, which occurred in Somalia in 1977. The WHO officially declared smallpox eradicated in 1980.

Every laboratory except one in the United States and one in Russia subsequently agreed to destroy any samples they had, and the WHO in 1990 set a deadline for getting rid of the last two by 1993. But the United States balked in 1994 after revelations emerged that that the former Soviet Union had worked to develop smallpox as a biological weapon. Since then, both the United States and Russia have repeatedly postponed, citing concerns that Iraq, Iran, North Korea and others might be hiding the virus and the imperative to conduct more research.

Because of the successful eradication program, vaccination programs stopped worldwide, leaving most defenseless against the disease. The United States discontinued immunizations in 1972, so anyone born after 1967 is vulnerable.

In the wake of the Sept. 11, 2001, terror attacks, the United States stockpiled enough vaccine to inoculate every American against smallpox. But the vaccine is imperfect. AIDS patients, transplant recipients and others with weak immune systems can not safely use it. Two new drugs appear to be effective but have not yet undergone sufficient testing to win Food and Drug Administration approval. Scientists need the live virus to develop good animal models to test them, some say.