Biological agents were meant to be delivered using the same types of weapons as were chemical warfare agents; the only difference is that the weapon is filled with an isolate of the biological agent rather than the chemical agent. Similarly, dispersalof a biological agent cloud is governed by many of the same phenomena as is the dispersal of chemical agents. Natural vectors are an additional means of disseminating biological agents.

Particle Size

The effect of particle size is key to the effectiveness of an attack with a biological warfare agent. Large particles have several drawbacks that generally make them inefficient transmitters of disease. These particles tend to settle out of the air relatively rapidly, limiting the time that they are effective. Once inhaled, large particles are most likely to impact on the mucous coat of the pharynx or nasal cavity. Although still capable of causing infection, the probability of infection decreases dramatically. Small particles remain airborne for a relatively long time; however, once they are inhaled, they are promptly exhaled. Small particles do not remain in the lung to cause infection. The most particles most likely to cause infection are those of an intermediate size. These particles remain airborne, settling relatively slowly, but one they are inhaled they reach the alveoli in the lung and remain there, with a large probability of causing infection.

One hurdle to military use of biological weapons is that an aerosol of ideally sized particles remains airborne for a relatively long period. It is subject to changes in the wind, which can potentially blow the cloud back at the attacker, and weather, which can wash the agent out of the atmosphere.

Natural Vectors

Although most biological weapons were designed to function via inhalation, biological warfare agents can also enter the body through contact or ingestion. In addition, agents can be spread through natural vectors:

• Arthropods, which are hard-shelled invertebrates with segmented limbs
  • Insects, including fleas, ticks, and mosquitoes
  • Crustaceans, including crabs, lobsters, shrimp, and barnacles
  • Arachnids, including spiders
  • Myriapods, including centipedes
• Contact with infected animals
• Contamination of food and water

Many agents also cause natural outbreaks of disease through these vectors.