Researchers are still identifying micro-organisms that cause new diseases in humans. A basic factor in the emergence of these diseases is the role played by animals, which act as a reservoir for certain viruses. In favourable conditions, such viruses can cross the species barrier and infect humans. The book takes a close look at two families of virus: orthomyxoviridae and paramyxoviridae, which have infected many species of vertebrates and are responsible for zoonosis. The two main parts of the book describe how the viruses operate, how they spread and the risk factors for humans. In addition to a specific and detailed study of these two micro-organisms, the book highlights the fascinating history of such diseases, their emergence, development and disappearance. They have occurred throughout human history, underscoring the role of the environment and the way it changes, often as a result of human intervention. Sponsored by the Mérieux Foundation and written by leading international specialists, this book provides first-class information about these new viruses.