The Parapoxviruses represent a genus within the chordo-poxvirus subfamily of Poxviridae, and are characterised by their ability to cause disease in livestock, semi-domesticated species, wildlife and humans. ruminants and humans. The genus currently contains four recognised species of virus, orf virus, the prototypic species, that infects sheep and goats, bovine papular stomatitis virus and pseudocowpox virus both of which infect cattle, and parapoxvirus of red deer in New Zealand. Comparatively to the more extensively studied poxviruses, vaccinia virus, little is known about the genes within the PPV that affect the host range and virulence. There is already preliminary evidence that viral genomic variations may account for the differing virulence of disease outbreaks. The evolution of poxviruses has been proposed to be an adaptive process involving frequent events of gene gain and loss, such that the virus co-evolves with its specific host. Gene capture or horizontal gene transfer from the host to the virus is considered an important source of new viral genes including those likely to be involved in host range and those enabling the virus to interfere with the host immune response to infection. It has also been shown, in other poxvirus genera, that recombination between isolates and perhaps even between species does occur, thereby providing another mechanism for the acquisition of new genes and for the rapid evolution of viruses. Such events may result in viruses that have a selective advantage over others, for example in re-infections, or in viruses able to jump the species barrier and infect new hosts.