The objective of this research was to quantify the likelihood of PRRSV transmission via aerosols as a function of exposure dose. This information is important because it reveals the process by which PRRSV maintains itself in swine populations. Understanding this process can lead to the development of more efficient methods of controlling transmission. The current work and comparison with previous work done in our laboratory led to two conclusions: 1) the infection dose for airborne PRRSV isolate MN-185 is very low; 2) the infectious dose of airborne PRRSV differs among isolates.