Porcine Reproductive and Respiratory Syndrome virus (PRRSv) and Porcine Epidemic Diarrhoea virus (PEDv) are two of the major viruses that affect pigs worldwide. This chapter focuses on the transmission, clinical presentation, evolution and spread of these two agents (PRRSv and PEDv), due to their importance in pig production worldwide and their evolution over the years. The chapter discusses in detail the agent, transmission, clinical presentation and evolution and spread of these two viruses in the pig population both worldwide, and more specifically, the United Kingdom. The chapter provides readers with an overview of the complexity of these two agents and how this influences their clinical presentation and evolution over time and space.