Similarly, dispersal of 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.
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.