It has been described some causes that promote the emergency or re-emergency of viral infections. A fundamental factor is the property of viruses to accumulate mutations which may help them to escape from the host immunological system, increasing their virulence or inducing resistance to antivirals. Nevertheless, there exist factors inherent to the human activity that play important roles in the emergency or re-emergency of viruses. These activities are related with traffic of exotic animals, deforestation, migration, introduction or development of human communities in unexplored areas and the climatic changes. Due to the great significance of emerging viruses as a public health concern, it is necessary to know different aspects of these phenomena, to take the necessary steps to prevent or control epidemics or pandemics.