Prof Hay: "Anthrax vaccine is very effective"

A second way would be for it to be supplied by a biological warfare programme - "it's difficult to know anything more about that because it is clandestine," Dr Hay said.

A third way, and one which Dr Hay thought would be more likely, would be for someone to steal a so-called reference collection.

These are cell cultures kept in hospitals or public health institutions, for checking suspected cases of the infection.

"There's one in most countries, and quite a few in several countries," he said. "And I understand that some are better controlled than others."

Aerosol problems

Anybody wanting to spread the spores in an office or underground train would have quite a difficult job, Dr Hay pointed out.

Anthrax is kept in several hospitals and laboratories

The Aum Shinri Kyo cult which killed 12 people on Japan's underground system in 1994 had failed at least half-a-dozen times to infect the system with anthrax.

Dr Hay said this was either because the strain was not virulent enough, or because they had problems getting it airborne.

"Inhalation anthrax is not a significant problem. It must be distributed in a very fine particle spray and that sort of technology is not generally available," he said.

Contaminating surfaces such as seats or keyboards was a different matter, however - it would be relatively simple to infect small numbers of people in this way.

But the dangers in contracting anthrax through the skin, rather than through the lungs, are much lower, said Dr Hay.

The symptoms - which begin with redness around the entrance point of the infection, followed by blistering - can be picked up very quickly.

The infected person can then be treated with antibiotics. Penicillin is effective in many cases.

Prophylaxis and vaccines

If a case has been confirmed in an office, public health officials can take swap samples to see if there are any spores left in the building.

It would be difficult to spread respiratory anthrax on an underground system

In the unlikely event that there are, Dr Hay said, workers can be put under surveillance for any symptoms, or put under preventative antibiotic treatment.

It would be possible to vaccinate workers if there were fears that a particular office was to be targeted, he said.

The vaccine had proved "very effective" for skin anthrax, although Dr Hay said there had not been enough cases to comment on its effect on respiratory anthrax.

"Here we're entering slightly unchartered territory," he said.