A nagging fear lurks behind the increasing threats to bomb the remnants of Saddam Hussein's military machine: that a missile hidden somewhere in the Iraqi desert could dump tons of nerve gas or deadly anthrax spores on the population of Tel Aviv, or a dozen other cities within range.

True, it is only a remote possibility. Far more remote than it seemed during the 1991 Gulf war, when the Scuds actually were falling on Tel Aviv and Riyadh.

But Saddam has shown he has no qualms about gassing his enemies, even his own citizens. Years of painstaking UN inspections have failed to account for at least two of the Iraqi Scud missiles not used in the Gulf war.

And, as an intelligence assessment from Whitehall revealed earlier this week, the UN inspection body, Unscom, simply does not know how many usable chemical or biological warheads lie hidden. Only in the past few months, according to Britain's Defence Secretary, George Robertson, a hitherto unmentioned chemical weapon - Agent 15 - has been identified in large quantities.

Whatever the real threat, it is plausible enough to send Israelis once more running for their gas masks. Elsewhere, scientists and intelligence agents - who for years have been emphasising the threat of nuclear proliferation - are turning their attention to the dangers of chemical and biological warfare.

The reason is simple. In the words of Prof Paul Rogers, head of the department of peace studies at Bradford University, England, "Nuclear weapons are far more difficult to produce than chemical and biological weapons. Any country with a reasonable agricultural industry can modify their sprays and dusts very easily to make CB weapons."

Also, as Unscom's experience in Iraq has shown, it is more difficult for states to hide their nuclear procurement activities than their CB warfare capability.

John Deutch, then CIA director, warned in 1996 that the proliferation of chemical and biological weapons in the hands of states and terrorist groups was "the most urgent, long-term pressing intelligence challenge that we face . . . The materials and expertise necessary to build chemical and biological weapons are more readily available today than ever before."

But it is one thing to produce CB weapons, quite another to devise effective delivery systems.

According to Unscom, Iraqi scientists have been conducting experiments to find out the most viable warheads and missiles to deliver CB agents.

Iraq has been experimenting with pilotless aircraft and a specially adapted MIG 21. Ten pilotless drones were discovered after the Gulf war in a bomb shelter at the headquarters of the Nair State Establishment for Mechanical Industries 1.

But Saddam's Iraq has not restricted itself to experiments. In March 1988 it used chemical weapons against its own citizens, when the Kurdish town of Halabja was attacked by aircraft; an estimated 8,000 civilians died.

The Iraq-Iran war also gave an opportunity to use gas to deadly effect on the battlefield. Now, according to Western intelligence sources, Iraq is even exporting the technology: last month they claimed that Iraqi scientists were helping Libya develop a biological weapons programme, based at the innocuous-sounding General Health Laboratories near Tripoli.

BIOLOGICAL and chemical weapons are nothing new. Romans poisoned wells by dropping corpses down them, an early form of areadenial. In 1346, the Tatars catapulted plague-infested corpses into the walled city of Kaffa and shortened what looked to be a long siege. Some historians argue that this may also have brought the Black Death to Europe.

Britain has not been averse to developing such weapons. During the first World War, Britain stockpiled five million cattlecakes infected with anthrax to drop on German cattle if the Kaiser's scientists used biological weapons.

At the close of the war, British, American and Canadian teams worked on an `anti-personnel' anthrax bomb which was never made 2. In fact, the Germans had already discovered - with mustard gas and chlorine attacks - that some weapons were simply too indiscriminate to be trusted.

Anthrax is an old enemy, mostly threatening those who handle animals, or animal products such as hides. It can be cured by drugs, and vaccines exist to protect vets, woolcarders and others at risk.

The anthrax bacillus is hardy, and its spores can hang around almost indefinitely. Yet that carries problems: early British anthrax experiments meant that the Scottish island of Gruinard had to be sealed off for decades.

In 1979, 96 people fell sick and 64 died in an anthrax outbreak in Sverdlovsk, in the then USSR. The Russians at the time said it arose from contaminated meat. Later, it became clear that there had been an explosion in a military biological weapons facility nearby.

The Japanese during the second World War conducted a series of experiments in the notorious camp 731 in Manchuria: they tested prisoners with botulism, encephalitis, typhoid, smallpox and 16 other microbes. After the war, the US developed weapons that used anthrax, yellow fever, tularemia, brucellosis and other fevers.

The military disadvantages of biochemical weapons - a danger to one's own troops as well as the enemy's - led to agreements to limit their use. The use of gas on the battlefield was outlawed under agreements dating back to the horrors of mustard gas during the first World War.

A Biological Weapons Convention was signed in 1972 by the US, the USSR and the UK. Yet the convention has not proved wholly successful: a 1993 assessment by a US Congressional office declared that Iran, Iraq, Israel, Libya, Syria, North Korea and Taiwan could have undeclared offensive biological-warfare programmes.

The Chemical Weapons Convention took a lot longer, held back by mistrust between Washington and Moscow. It did not finally come into force until last April.

Destroying the old weapons was always going to be slow, difficult and expensive. And by this time disillusionment had set in as to how far the arms-control regime could really reach.

But what alarmed the strategic analysts was the combination of rapidly proliferating ballistic missiles, especially the ubiquitous Soviet-built Scuds, and warheads potentially filled with anthrax or VX nerve agent.

Syria, for example, is not seeking nuclear weapons as far as we know. But it does have chemical weapons, and plenty of Scuds.

The great concern nowadays, however, is not simply the use of such weapons by a rogue state like Iraq. There is increasing concern about the threat from terrorist groups.

CB weapons, used either against carefully-chosen targets or at random, cannot be controlled by conventional military action.

The first large-scale chemicalweapons attack by a non-state group took place in March 1995 in Tokyo. Members of the Aum Shinrikyo religious sect released sarin nerve gas on the subway system, killing 12 and injuring 5,500.

A droplet of sarin on the skin, or inhaled, renders the victim incapable, and soon dead. The sect had attracted a number of experienced scientists who, according to Japanese police investigators, were also experimenting with other substances, including anthrax.

It could have been worse. In 1992, the head of the Aum cult went to Zaire, ostensibly to help Ebola virus victims - but a US Senate report says it was to get samples3. Ninety per cent of Ebola victims die, horribly, within a week.

There is evidence of other plans to use CB attacks. In 1995, an Ohio man was arrested trying to buy bubonic-plague cultures through the post. A year later, German police seized from a neoNazi group a coded disc with information about how to use mustard gas.

Dr Alistair Hay, a microbiologist at Leeds University, England, began warning of bio-weapons in terrorists' hands more than 20 years ago. But even the convention's signatories contain rogue forces.

"One thing that staggered me was the biological-warfare work on plague going on in Russia in 1992. The government was saying one thing, and the KGB were running a different operation. One wondered how much control some of these countries have over different rogue outfits."

If that worries you, there's worse to come. The new science of genetic engineering raises a hazard to yet higher notches: the fear is that unscrupulous scientists could engineer even more lethal poisons or more virulent microbes.

Yet work is going on, everywhere in the world, on the re-engineering of microbes for commercial and medical reasons. How can governments detect evil manipulation at literally microscopic scales?

Sources: (1) CIA; (2) Biological Weapons: An Increasing Threat, by Wendy Barnaby (Medicine, Conflict and Survival, Vol 13, No 4, 1997); (3) The Spectre of Biological Weapons, by Leonard A. Cole (Scientific American, 12/1996).