Responsibility for safeguarding the stockpiles lies with men like Sergei Netesov, Vector's deputy general director.

"We feel it, very heavily," the soft-spoken, bespectacled Russian scientist told Reuters in an interview at a security conference in Berlin.

"We look after very dangerous viruses and try to work out new ways to combat them. It's morally right, and people are proud of that kind of work."

20TH CENTURY KILLER

Beginning with a rash and developing into pustules that spread across the entire body, leaving permanent pitted scars, smallpox killed an estimated 300 million people in the 20th century alone.

It had an overall mortality rate of around 30 percent until it was declared eradicated in 1980 following a worldwide vaccination programme.

Scientists believe the death rate if it re-emerged today would be much higher because vaccination - which carries a risk of side-effects - is no longer widely practised.

The only official stockpiles of the virus are held at Vector, near the Russian city of Novosibirsk, and at the U.S. Centers for Disease Control in Atlanta.

Netesov argues strongly that they must be preserved, not destroyed, in order to study the origins of the virus and its genetic blueprint, and to seek new vaccines and anti-viral agents. There is still no known treatment for the disease.

"I think in reality we'll always have questions about nature, including about this virus. On the other hand, I think it's realistic to complete basic research in the next 10 or 15 years," Netesov said.

But by that time, he went on, genetic engineering would have advanced to the point where scientists could create new smallpox strains synthetically in the lab. "When that develops, there won't be much sense in destroying the stocks."

BIO-TERROR WEAPON?

Did that mean that terrorists could also manufacture the virus?

"The technology is very complex. For bio-terrorists it would be easier to steal it than to make it, because to conceal a well equipped laboratory is hardly possible for a long time," Netesov said.

"The threat we need to worry about most is theft, and probably not by someone coming from outside, but theft by someone working in a laboratory. Therefore the system of selecting people has to be very thorough."

Western analysts have often voiced concern that secrets or weapons technology could be bought or smuggled out of former Soviet research centres with highly qualified, low-paid staff.

Netesov said Vector staff typically earned $200 to $300 a month, several times higher than the Russian average.

He insisted security was tight around the installations where scientists study smallpox, SARS, Ebola, Marburg and other deadly viruses.

HUMANS ARE WEAKEST LINK

Armed police patrol the perimeter and visitors are subjected to two tiers of security checks. Staff, who are trained in bio-ethics, must have worked at least three years at the centre before they are allowed to handle the most dangerous viruses.

"The weakest link is human beings. We have to check a person very closely before we trust him."

Unlike, for example, anthrax, which cannot be transmitted from person to person, smallpox is highly contagious and can be spread via droplets in the breath, skin-to-skin contact or infected clothing or bed linen.

The Soviet Union used smallpox in its biological weapons programme, but Netesov said he believed suggestions that germ warfare experts had developed especially virulent and deadly strains were "fairytales". He also thought it highly unlikely that countries like North Korea had developed smallpox as a bio-weapon.