Virus evolution: disease emergence and spread

Many viruses with RNA genomes have genetically diverse populations called quasispecies. The representation of any particular sequence within this quasispecies is a result of interactions between the host and environmental factors affecting the replication of the virus. Important biological properties are a direct result of the levels of diversity in the quasispecies 'cloud size', including adaptability and host range. RNA viruses have become the model system for the analysis of viral evolution due to the inherent error-prone nature of their genome-replicating enzymes that lack a proof-reading function. However, these viruses have also risen in their importance, due to the nature of the diseases that they inflict upon man as well as the crops and animals on which we are dependent. The emergence of previously described viruses into new geographic locations or of previously undescribed viruses into a new host or environment can be explosive and have far reaching consequences. Recent examples include human immunodeficiency virus, West Nile virus, Newcastle disease virus, bluetongue virus and henipaviruses. This paper describes some recent incidents and our state of knowledge in this area