THERE IS NOTHING PERMANENT EXCEPT CHANGE - THE EMERGENCE OF NEW VIRUSDISEASES

The sudden appearance of apparently new viruses with pathogenic potent ial is of fundamental importance in medical microbiology and a constan t threat to humans and animals. The emergence of a ''new'' pathogen is not an isolated event, as for instance the frequent appearance of new influenza virus strains demonstrates. Often the new virus strains co- circulate with the older strains in a susceptible population, but a re placement of the older strains has been also observed. In rare instanc es the new viruses can cause dramatic epidemics or pandemics, such as those observed with the human immunodeficiency virus, canine parvoviru s, or most recently, with the agent of bovine spongiform encephalopath y in the United Kingdom. The mechanisms of the emergence are not alway s clearly understood, but an altered host range appears to be a common event. Whether a true change in host range occurs, or whether the vir us adapted to the host and replicated more efficiently, is often unkno wn. This review tries to summarize the facts that are known about a wi de variety of ''new'' viruses of mammals, such as the simian, human an d feline lentiviruses, the feline coronaviruses, the feline parvovirus es, the carnivore morbilliviruses, the influenza A viruses, and the tr ansmissible spongiform encephalopathies. A particular emphasis will be put on the genetic mechanisms that might have taken place and that mi ght have been responsible for their sudden appearance.