Viruses, the simplest organisms, obligate parasites that encode structural proteins and replicative functions requiring the cellular machinery of the host to ensure their propagation. Viruses are masters of evolution. An analysis of infectious diseases emerging since the 1980s revealed that most were caused by viruses, especially those with RNA genomes. New viral emergences are generally the result of intrinsic changes in the genetics of the virus to increase transmission, virulence or host range, and environmental or ecological changes that favor contacts between viruses and humans or other hosts. We describe the molecular mechanisms of viruses that ensure their rapid adaptation and evolution. We describe instances where viral evolution appears partially responsible for recent outbreaks and discuss the challenges in identifying the cause or consequence of viral evolution in the context of resource-rich versus -limited countries.