Compared to nuclear, chemical, or conventional weapons, biological weapons are unique in their diversity. Dozens of different agents can be used to make a biological weapon, and each agent will produce a markedly different effect. These differences are shaped by various properties of the particular agent, such as its contagiousness, the length of time after release that it survives in the environment, the dose required to infect a victim, and of course the type of disease that the agent produces.
Biological weapons are relatively inexpensive and easy to produce.
Although the most sophisticated and effective versions require considerable equipment and scientific expertise, primitive versions can be produced in a small area with minimal equipment by someone with limited training. Since industrialized countries have adequate water purification systems, contamination of the water supply is the least effective method for disseminating a biological weapon in such countries. Contamination of food supplies would most likely be used in a terrorist attack, since it is difficult to contaminate enough food to gain a military advantage. Release of infected vectors is not particularly efficient for either military or terrorist purposes and entails a high probability of affecting those producing the weapons or living nearby. By far, the most effective mode for applying biological weapons is an aerosol cloud.