Porcine reproductive and respiratory syndrome (PRRS) is an economically important disease of swine resulting from the infection with a single-stranded, positive-sense RNA virus called porcine reproductive and respiratory syndrome virus (PRRSV). First recognized in the United States in 1987 and subsequently detected in Europe and other parts of the world, PRRS is characterized by late-term reproductive failure in sows (e.g., infertility, abortions, stillbirths, or the birth of weak piglets) and respiratory distress in piglets. With the capacity to induce severe clinical disease and maintain a life-long subclinical infection in swine population, PRRSV is a serious threat to the swine industry worldwide. In the United States, the estimated cost associated with PRRS amounts to >600 million U.S. dollars per year.