Since the discovery of the PRRS virus there have been large strides in further understanding it and how it moves within  a swine herd, from one herd to another and how it is diagnosed. This article by Scott Dee summarizes the facts that we currently know about the PRRS virus. Most pigs are infected with the PRRS virus from an infected sow. As sows give birth to piglets they become infected while lactating and run the risk of transferring the virus to older pigs who were not born with PRRS but grouped together with these PRRS positive pigs. Understanding this transmission sequence is crucial in achieving a stabilized herd, meaning there is no chance of transmitting the virus from sow to sow or sow to pig. The work to eliminate PRRS from swine herds has been well documented, despite this completely PRRS negative herds still become infected with the virus. This was originally blamed on introducing PRRS positive pigs into  a PRRS negative population and the use of semen that my contain the PRRS virus. But further research has proven that PRRS can also be transmitted by fomites, insects, in transport trailers and aerosols. Many new forms of diagnosing PRRS have become available since the discovery of the virus including the IDEXX ELISA, polymerase chain reaction and nucleic acid sequencing. Although each method still has drawbacks such as it can be difficult to interpret data for an individual pig.