The US Defence Secretary, William Cohen, provides an analogy

It is a deadly disease that can be spread by people breathing it in, it kills 90% of those infected and it exists in tiny quantities in cattle grazing areas. Potential terrorists can produce anthrax without access to exotic chemicals.

Books explaining how to manufacture a variety of poison gasses and other toxins can be bought in the US or ordered across the Internet.

According to the report to the US Congress' former Office of Technology Assessment, an aircraft releasing 100kg of anthrax over a large city on a calm, clear night could kill between one and three million people.

Fortunately, according to Dr Jonathan Tucker, an expert in chemical and biological weapons who served on a UN weapons team in Iraq, using such weapons in a terrorist attack is not easy.

"Trying to produce 100,000 casualties is much more difficult than is often stated", he said.

In March 1995, members of the Aum Supreme Truth cult in Japan made several poison gas attacks on the Japanese subway system - the first large-scale terrorist use of chemical or biological weapons.

While 12 people were killed and thousands sent to hospital, equipment failures and human error kept casualties from being much higher.

Dr Tucker adds it can be difficult to design ways to deliver these toxins, and weather conditions and impurities in the toxins themselves can frustrate a terrorist attack.

Anthrax - its effects

Anthrax is a bacterium, spread by spores which can be breathed in, or by skin contact. These spores are present in cattle and sheep grazing areas but at such low levels that farm workers are normally safe from infection.

It is not spread from person to person, though medical professionals recommend that the clothing and other soiled articles of the infected should be burned.

Those most at risk can be vaccinated - some textile workers are, for example, and in December 1997 it was announced that all US troops were to be vaccinated in a rolling programme.

Those infected can also be cured with an antibiotic if it is administered within 12 hours of infection - penicillin will work, though penicillin-resistant strains of Anthrax exist.

If it is inhaled, symptoms include mild fever, malaise, fatigue, coughing and, occasionally a feeling of pressure on the chest. If it has been contracted through the skin, there will be a boil, which eventually forms a black centre.

Unfortunately, these symptoms can be missed or mis-diagnosed, and sometimes they do not start until after the 12 hour period when treatment is effective.