By Imogen Foulkes BBC News, Geneva

The last smallpox case was in 1977

The World Health Organisation is to consider the controversial question of whether to permit further research on the smallpox virus.

Many scientists claim a genetic modification of the virus could speed up the development of new treatments against the disease.

But critics say that the best way to defeat smallpox is to destroy the last remaining samples of the virus.

Smallpox was eradicated in 1977, a major victory for the WHO.

Finding a balance

Smallpox is one of the deadliest diseases the world has ever known.

It kills over a quarter of all of those infected with it, and those who survive are left disfigured and often blind.

As late as the 1960s, smallpox claimed over two million lives a year.

After its eradication, the next step - to destroy all remaining laboratory samples of the virus - was promised, but never taken.

Dr Mike Ryan, director of the WHO's alert and response office, believes research on the virus will help to defeat any new outbreaks.

"We need to be absolutely sure that we have the necessary antivirals and the necessary vaccines that we need to fight this virus should someone release this from another source," he says.

"I think this is the continual balance that the member states of WHO have to face."

Fluorescent protein

The proposal for genetic modification will be heard by the World Health Assembly, which brings together all 192 member states of the World Heath Organisation.

The debate was originally planned for Wednesday, but is now expected by the end of the week.

This plan involves inserting a fluorescent protein into the virus.

It will grow green if the virus is alive, but won't show if it is dead, so it is claimed it is a fast and safe test for antivirals.

But critics of this research don't agree.

They say it will open a Pandora's box of genetic manipulation of dangerous viruses and may even end up defeating the WHO's stated purpose, which is to be ready with treatment should smallpox be released.

They point out that the last person to die from smallpox did so after the virus escaped from a laboratory.