Abstract: Porcine reproductive and respiratory syndrome (PRRS), which has also been termed mystery swine disease, swine infertility and respiratory syndrome (SIRS), and porcine epidemic abortion and respiratory syndrome (PEARS), induces severe disease in pigs and causes considerable economic loss to farmers. The pathology of PRRS is characterized by severe reproductive failure in sows, mild to severe respiratory distress, increased mortality in weaning pigs, conjunctivitis, and lymphnode enlargement. The pathogen responsible for PRRS is an enveloped RNA virus belonging to the Arterivirus group within the Togaviridae family. The PRRS viruses are single stranded RNA viruses having a viral genome of positive polarity and a size of approximately 15 kb. The positive-strand RNA genome possesses at least three major structural proteins designated N, M, and E. The PRRS viruses exist as a quasispecies and display considerable genotypic and phenotypic heterogeneity.