London (Reuters) - Former Soviet germ warfare scientists have called for worldwide vaccination against the deadly virus after admitting poorly paid Russian scientists could be tempted by rouge states which want their expertise in the deadly pathogen.

Front-line medical workers in the United States were also vaccinated against the virus as precautionary measure in case of an attack with smallpox, which was declated in 1981 to have been eradicated worldwide by mass inoculation.

It is one of the most feared biological weapons because of this potential speed of spread and fatality rate. Transmissible by inhaling virus particles, it is highly infectious, unlike anthrax.

"It is spread by sharing same airspace," Dr. Brian Durden, the medical director of Britain's Public Health Laboratory (PHLS), which monitors infectious diseases, said in an interview.

The United States and Russia have the only smallpox, which are held under tight security. Scientists fear countries such as North Korea and Iraq might also have stocks of the virus.

Lev Sandakhchiyev, the director of Russia's Vektor Institute, which holds the Russian samples, told journalists on Monday that the world health system was unprepared to deal with a major outbreak.

"All you need is a sick fanatic to get to a populated place," he said at a news conference.

Donald Ainslie Henderson, a former director of the World Health Organization Smallpox Eradication Unit, which is credited with eliminating the disease, once estimated it would take only about six weeks to seed smallpox cases around the world.

"Dropping an atomic bomb could cause casualties in specific areas, but dropping smallpox could engulf the world," he told author Richard Preston in New York magazine interview in 1999, when bioterrorism was still in the realms of Science fiction.

After the September 11 attacks in New York and Washington and with four deaths in the United States from anthrax, the threat has suddenly become real.

"If a terrorist organization ever get hold of smallpox it would be dreadful. It is a highly virulent, highly contagious virus," said Simon Whitby of the department of peace at Bradford University in northern England.

"Literally, it has caused millions of deaths over the years," he said.

Duerden of the PHLS said the only theraphy against smallpox is prevention by vaccination. Those infected can also be vaccinated within four days of exposure to the virus, he said.

Most doctors have never seen a case of smallpox and would have difficulty diagnosing it quickly, which has prompted health officials on both sides of the Atlantic to issue education aids to help doctors recognize it.

Smallpox produces flu-like symptoms and a distinctive and disfiguring rash. The disease has left its mark on the English language with words such as pox mark, which refers to the scarring, and poxy, meaning poor quality or worthless.

The incubation period is usually around 12 days. Symptoms include tiredness, chills, fatigue and fever followed by unsightly pustules that erupt mainly on the face and limbs.