Fears that terrorists could use the smallpox virus for mass casualty attacks have led both the United States and Russia to retain stocks of the lethal smallpox virus for medical research.
The disease was declared eradicated in 1980, and the World Health Organisation had called for the destruction of the world's stocks of the smallpox virus by June, 1999.
But the pathogen is still held in secure facilities at the State Research Centre of Virology and Biotechnology, or VECTOR, in the Siberian city of Kotsovo, and at the Centres for Disease Control and Prevention in Atlanta.
President Bill Clinton reversed an earlier US government pledge to destroy the stocks after experts warned samples held at VECTOR might have leaked into the hands of terrorists or rogue states in the chaotic years after the Soviet Union collapsed.
Their concerns centred around claims made by Ken Alibek, a former VECTOR official who defected to the United States in 1992.
Mr Alibek said that the Soviet Union had succeeded in splicing genes from other viruses to produce smallpox microbes that would be invulnerable to vaccines.
"Given the extensive research in which the Soviet Union was engaged, there were reasons to believe there may be other facilities in Russia where stocks are held," said Amy Smithson, a disarmament specialist.
Experts said that the US needed to retain its stocks of the smallpox virus to combat future outbreaks, either caused by accident or rogue actors.
Last year, the US Centres for Disease Control said that after 9/11, there was concern that the virus could be used "as an agent of bioterrorism." Last year, a group of 40 al-Qaeda terrorists were reported to have died of plague in Algeria, after a biological weapons experiment went wrong.
The US Institute of Medicine at the National Academy of Sciences also argues the virus could lead to "new and important discoveries with real potential for improving human health." New vaccines against smallpox continue to be produced. Project Bioshield, a US government pandemic preparedness programme, last year received twenty million doses of smallpox vaccine for immune-compromised populations.