There is an interesting paradox when it comes to war in the modern world. Anyone who has experienced war knows that it is about death and destruction on a massive scale. People die one at a time because of bullets, bayonets, hand grenades and landmines, and they die in large groups because of cannons, bombs and missiles. Buildings, factories or entire cities get destroyed.
Despite the appearance of anarchy, warfare between modern nations does have rules. These rules, for example, tend to discourage the wholesale destruction of civilians, and they govern the treatment of prisoners of war. The rules are not always followed to the letter, and many times are broken completely, but they do exist.
Chemical weapons were first used in World War I, and the nations of the world quickly and uniformly decided that these weapons went too far. Apparently, killing people with flying metal and explosives was one thing, but launching a cloud of deadly chemicals -- the effects of which could neither be predicted nor controlled -- was another. Significant treaties prohibiting biological and chemical weapons, starting as early as the 1925 Protocol for the Prohibition of the Use in War of Asphyxiating, Poisonous or Other Gases, and of Bacteriological Methods of Warfare, have been signed by most nations of the world.
The unfortunate problem is that terrorists, and rogue nations like Iraq, don't pay attention to significant international treaties. That is where the threat of chemical and biological weapons used in random attacks on innocent civilian populations comes from.