WASHINGTON -- Fearing a biological attack from Iraq, the United States has made new smallpox vaccines and readied response plans for the reappearance of the deadly virus, which some experts are convinced is part of Saddam Hussein's arsenal.

Clues include U.N. weapons inspectors' discovery of a machine labeled "smallpox" and Iraq's experimenting with a related virus that infects camels. The official U.S. position, shared by some experts, is that the evidence is inconclusive.

"I don't believe the intelligence community has a smoking gun that Iraq possesses the virus," said Jonathan Tucker, a former U.N. biological weapons inspector. "My impression is they're erring on the side of caution on these bits of circumstantial evidence that are troubling but not conclusive."

Chief suspect

All smallpox samples except one in Atlanta and one in Moscow were supposed to have been destroyed more than two decades ago when the disease was declared eradicated. The former Soviet Union reportedly grew tons of smallpox as part of its biological weapons program, and experts suspect other countries could be harboring clandestine samples.

Chief among the suspects is Iraq, which has admitted making weapons from anthrax and other germs.

"I have no doubt in my mind that Iraq does have the smallpox virus," said Dr. Ken Alibek, a top official in the Soviet biological weapons program before he defected to the United States in 1992.

An attack with smallpox could kill millions, a possibility that prompted the Bush administration to order enough smallpox vaccine to inoculate the entire U.S. population if necessary.

"We're very worried about Iraq," said Dr. D.A. Henderson, a smallpox expert and bioterrorism adviser to the Department of Health and Human Services.

Unlike anthrax, the bacteria used in last year's unsolved mail attacks, the highly contagious smallpox virus can be passed from person to person. The virus causes ugly pustules both on the skin and inside the mouth and throat. About a third of unvaccinated people who get the disease die.

Protection may fade

The last case of smallpox in the United States occurred in Texas in 1949, and routine vaccinations ended in America 30 years ago. That means at least two out of five Americans have not been vaccinated, and studies suggest the vaccine's protection probably fades over time.

U.N. weapons inspectors and U.S. intelligence agencies have found several clues suggesting Iraq might have the smallpox virus.

In 1994, U.N. inspectors at an Iraqi medical complex found a freeze-dryer labeled "smallpox" in Arabic, Tucker said.

The Iraqis said the equipment was used to make smallpox vaccine, Tucker said. A freeze-dryer also could be used to make a weapon-ready form of the smallpox virus.

"It's not conclusive proof but suggestive of Iraqi interest," said Tucker, author of a recent book on smallpox.

Iraq also admitted to U.N. inspectors that its biological weapons scientists worked with camelpox, a close relative of the smallpox virus that doesn't usually infect people. Working with camelpox would give Iraq a way to perfect techniques for making smallpox weapons without endangering the researchers.

"The only explanation is they used it to see how to grow smallpox, how to concentrate it, how to deploy it. It's a perfectly good and safe model for this," said Alibek, now director of the George Mason University Center for Biodefense in Manassas, Va.

"It's hard to believe Saddam would do this work to protect his camels."

Tests on Iraqi soldiers captured during the 1991 Persian Gulf War found that some had been vaccinated for smallpox, according to a declassified Defense Intelligence Agency report. That could be evidence that Iraq was trying to protect some of its soldiers in the event it used a smallpox weapon.

Iraq's last reported outbreak of smallpox came in 1972, the same year Iraq started biological weapons research. A World Health Organization report estimated that about 800 people were infected in that outbreak, which had spread from Iran and later spread to Syria and Yugoslavia.