US government workers cleaning out an old storage room at a research centre near Washington, USA, made a startling discovery last week - decades-old vials of smallpox packed away and forgotten in a cardboard box.
The six glass vials of freeze-dried virus were intact and sealed with melted glass, and the virus might have been dead, officials at the Centers for Disease Control and Prevention said Tuesday.
Still, the find was disturbing because for decades after the lethal disease was declared eradicated in 1980, world health authorities believed the only samples of smallpox left were safely stored in super-secure laboratories in Atlanta and in Russia.
Officials said this is the first time in the US that unaccounted-for smallpox has been discovered.
It was the second recent incident in which a government health agency appeared to have mishandled a highly dangerous germ. Last month, a laboratory safety lapse at the CDC in Atlanta led the agency to give scores of employees antibiotics as a precaution against anthrax.
The smallpox virus samples were found in a building at the National Institutes of Health in Maryland that has been used by the Food and Drug Administration since 1972, according to the CDC.
Officials said the vials may have been stored there since the 1950s - no records were found that said exactly when they were placed there.
No one has been infected, and no smallpox contamination was found in the building.
Smallpox can be deadly even after it is freeze-dried, but the virus usually has to be kept cold to remain alive and dangerous.
In an interview Tuesday, a CDC official said he believed the vials were stored for many years at room temperature, which would suggest the samples are dead. But FDA officials said later in the day that the smallpox was in cold storage for decades.
Both FDA and CDC officials said more lab analysis will have to be done to say if the germ is dangerous.
"We don't yet know if it's live and infectious," said Stephan Monroe, deputy director of the CDC centre that handles highly dangerous infectious agents.
The samples were rushed to the CDC in Atlanta and will undergo up to two weeks of testing to establish whether they are dead, Monroe said. Then they will be destroyed.
In at least one other such episode, vials of smallpox were found at the bottom of a freezer in an Eastern European country in the 1990s, according to Dr David Heymann, a former World Health Organization (WHO) official who is now a professor at the London School of Hygiene and Tropical Medicine.
Heymann said it is difficult to say whether there might be other forgotten vials of smallpox out there. He said that when smallpox samples were consolidated for destruction, requests were made to ministers of health to collect all vials.
"As far as I know, there was never a confirmation they had checked in with all groups who could have had the virus," he said.
Dr Donald Henderson, who led the WHO smallpox eradication effort and is now a professor at the Center for Health Security at the University of Pittsburgh, said it is highly unlikely more such stashes will be discovered. But he said "things were pretty casual" in the 1950s.
Decades ago, he recalled: "I came back from many a trip carrying specimens, and I just put them in the refrigerator until I could get them to a laboratory. My wife didn't appreciate that."
Smallpox was one of the most lethal diseases in history. For centuries, it killed about one-third of the people it infected, including Queen Mary II of England, and left most survivors with deep scars on their faces from the pus-filled lesions.
The last known case was in Britain in 1978, when a university photographer who worked above a lab handling smallpox died after being accidentally exposed to it from the ventilation system.
Global vaccination campaigns finally brought smallpox under control. After it was declared eradicated, all known remaining samples of live virus were stored at a CDC lab in Atlanta and at a Russian lab in Novosibirsk, Siberia.
The labs have the highest possible security measures. Scientists who work with the virus use fingerprint or retinal scans to get inside, wear full-body suits including gloves and goggles, and shower with strong disinfectant before leaving the labs.
The US smallpox stockpile, which includes samples from Britain, Japan and the Netherlands, is stored in liquid nitrogen.
There has long been debate about whether to destroy the known samples.
Many scientists argue the deadly virus should be definitively wiped off the planet and believe any remaining samples pose a threat. Others argue the samples are needed for research on better treatments and vaccines.
At its recent annual meeting in May, the member countries of the World Health Organization decided once again to delay a decision.