Image caption The viral disease was eradicated over 30 years ago

Defeating smallpox has been labelled as one of science's greatest success stories.

The disease once killed 30% of those infected, but after a global vaccination campaign it was declared eradicated in 1980.

However the variola virus, which causes the infection, is not gone. It exists in two laboratories, one in the US and the other in Russia. The question is about to be asked, once again: should they kill their stocks?

Smallpox facts Caused by the variola virus

The virus originated over 3,000 years ago in India or Egypt

Symptoms include fever, muscle pain, headache, tiredness and the distinctive rash

It killed 30% of those infected

More than 300 million people were thought to have died from smallpox in the 20th century alone

Up to 80% of survivors were marked with deep pitted scars, mostly on the face

The World Health Organization (WHO) will come to a decision at the 64th World Health Assembly this week.

It is not the first time the issue has arisen, it was first discussed at the Assembly in 1986 and has been the source of debate ever since.

Are stocks still needed?

Destroying the remaining stocks is seen in parts as the final chapter in eradicating the disease, otherwise there is always the risk of accidental release.

Others including the US and Russia argue for more research in case smallpox returns, possibly as a biological weapon.

They fear vials of the virus could exist outside of their labs. The genomes of around 50 strains of the variola virus have also been fully sequenced, and research has already shown that a virus can be built from scratch with such a blueprint.

Professor Geoffrey Smith, from Imperial College London, has been following the latest research on smallpox. He says studies have been focused on three areas - tests to diagnose the infection quickly and accurately, antiviral drugs to treat it and safer vaccines to prevent it.

He led a review of the state of scientific research on behalf of the WHO, which was published at the end of 2010, and concluded there had been "remarkable advances" in tests for smallpox.

Image caption Nearly one in three people who caught smallpox died

But the same could not be said with certainty for smallpox drugs and vaccines. While new candidates have been developed, they cannot be clinically proven as there are no human smallpox patients to test them on. Without trials to prove a medicine works, the endpoint for research becomes harder to define.

Professor Smith said: "It is fair to say the committee had mixed views on whether the research was there or nearly there but not quite."

Keep or destroy

The US secretary of health Kathleen Sebelius says it would be premature to destroy remaining stores of the virus now.

She restated in a column for the New York Times her country's commitment to eventually destroy stocks, but not yet.

She wrote: "We have more work to do before these safe and highly effective vaccines and antiviral treatments are fully developed and approved for use.

"Destroying the virus now is merely a symbolic act that would slow our progress and could even stop it completely, leaving the world vulnerable."

But the man who led the WHO's smallpox eradication programme from 1966 until the last case in 1977 disagrees.

Dr DA Henderson told the BBC: "I think it's a very good idea to destroy. At this point the reasons for keeping it are very obscure. Group after group has looked at this and basically said there is no need to retain it.

History of a vaccine In 1796, Edward Jenner extracted pus from a cowpox pustule on a milkmaiden and inserted it into a cut in the arm of an eight-year-old boy, James Phipps. He was working from rural folklore which said milkmaids who suffered cowpox never went onto develop smallpox. Jenner then proved that having been inoculated with cowpox Phipps was immune to smallpox. In 1959 the World Health Assembly passed a resolution to eradicate smallpox. The last natural case of smallpox was in Somalia in October 1977. In September 1978 Janet Parker, a medical photographer at the University of Birmingham died after being accidentally infected with smallpox. BBC History: Smallpox, Eradicating the Scourge

"We have done all of the productive research that we can do. It has been discussed fully and thoroughly by people around the world. Now is the time to destroy the virus as a further deterrent to anybody ever again producing it or using it."

Professor John Oxford, a virologist at Queen Mary University of London, believes the bioterrorism threat is "a load of old tosh" but still argues in favour of keeping the virus.

He said the decision was "tricky" but added: "I don't think there's a strong argument to destroy stocks, just an instinctive feeling to do it, which is misplaced.

"It's eradicating a whole species and you never know what the future might hold."

The argument holds no weight with Professor Gareth Williams, whose book - Angel of Death - charts the history of smallpox.

"There is no point in keeping it really. It has been sequenced completely so it can be recreated in a test tube and if it comes back you've got as much virus as you could want.

"It's just a vague sense of political unease keeping stocks, it has nothing to do with the scientific argument."

The vote

No one knows what will happen when health ministers from the WHO's 193 member nations discuss the issue.

Professor David Heymann, a former assistant director general for health security and environment at the World Health Organization, said there had historically been a split between the industrialised and developing countries.

He said developing countries have felt it is more important to deal with "known risks" than unknown risks like smallpox, while industrialised nations have different priorities.

Resolutions from the World Health Assembly are not legally binding, so the US and Russia cannot be forced to destroy stocks even if the majority of nations wanted it to happen.

The Assembly could reach a consensus agreement, such as postponing the decision, rather than forcing a vote because, as Professor Heymann puts it, "nobody wants to see the World Health Organization lose power" if its rulings are ignored.