PPRS is a detrimental disease to swine producers. To combat PRRS, systems are stocked with PRRS negative animals and semen. Despite this being a measure to prevent PRRS the introducti0n of these animals has been considered the most important source for virus introduction. Although new research has shown that fomites, insects and transport all can help transmit the PRRS virus. The objective of this study is to provide information on the epidemiology of PRRSV breaks in negative systems where the source of replacement animals and semen is negative and to illustrate the ease with which lateral infections occur. Thirty-five PRRS outbreaks were tested during a four year period. They broke down as such: 21 in sow herds, 6 in finishing sites, and 8 in boar studs. Herds were routinely monitored through the observation of clinical signs, blood sample collection and semen testing in the case of boar studs. The samples were compared to samples taken from neighboring sites, and in  a lab tests were done to see if the strains were from the same source. Attempts were made to determine the origin of the disease that were considered lateral, not being caused by introduction of new animals or semen. These infections were tested to see if they were because of area movement of the disease, pig movement with in the barn, transport, insect carriers, or human carriers. In this study 83% of infections were considered lateral, the remaining 17% were caused by introduction of infected animal or semen. Area spreading was considered to be the cause of 52% of the lateral cases. Transport due to the removal of cull sows, boars or slaughter and feeder pigs, was strongly suspected to be the causes of 17% of the lateral infections. Insects were suspected in one case, while bio-security breaches were suspected in two cases. This study provides insight into systems that have PRRS negative pigs brought into them. PRRS is then caused mostly by area, meaning vicinity to other operations and transport of dead animals.