In order to answer this question it is useful to be able to quantify the frequency of the transfer of genetic material between the virus and its host, as well as its impact on their evolution. And that is wherein lies the value of PhEVER (Phylogenetic Exploration of Viruses' Evolutionary Relationships), a new database which is useful as far as phylogeny is concerned, a discipline which aims to classify the genetic sequences which code for a protein and which have their origins in the genomes of different biological species according to their ‘homologies’ or degree of relatedness (or degree of similarity) with a view to understanding of this protein. The classification is set up in a database, in other words a structure which allows the storage of and access to a vast quantity of information in an efficient manner, in this case genetic sequences which come from the genomes of diverse living species and which have been decoded. Complex computer programmes then allow a comparison of the sequences which come from the same family of genes, on other words coding for proteins with similar structural or functional characteristics, in order to scan the homologies. The resulting classification is presented in the form of a ‘phylogenetic tree’ and represents the evolutionary history of this family of genes. If the topology of the tree, which thus shows the history of the protein over the course of evolution, is different from the known evolutionary tree of living species then the evolutionary history of the protein is different from that of the species. That indicates that the protein has not only evolved through direct line of descent (vertical hereditary transfer of the genetic material of one generation to the following one), but by exchanges between species of the same generation, what is known as horizontal transfer.