Emerging diseases are a major threat to modern societies, impacting individual welfare as well as economic development. The trend of newly emerging diseases has accelerated in the last two decades to such an extent that a new emerging infection is described at least once a year. The majority of such threats to modern society have been due to emergent viruses. This series of lecture notes provides grounding in understanding the drivers of disease emergence, the molecular processes which allow for virus diversity, the response of the host and environmental factors responsible for changing the balance between host and pathogen. Groups of viruses are described, each selected to illustrate certain features of disease emergence.