Biological and chemical weapons: What they are and what they do

Biological and chemical weapons are easier than nuclear arms to produce and hide, although they do require specialized scientific expertise in order to be effectively used.

Biological agents are odorless, tasteless and invisible to the human eye. Weight-for-weight, they are hundreds to thousands of times more potent than a chemical weapon, according to the Center for Defense and International Security Studies in the UK.

The Centers for Disease Control and Prevention in the United States lists six biological diseases as "high-priority" biological agents that pose a risk to national security.

The CDC also describes more than two dozen chemical agents that could be used as terrorist weapons.