An Ecological Framework of the Human Virome Provides Classification of Current Knowledge and Identifies Areas of Forthcoming Discovery

Parker, Michael T.

However, not all parasitic viruses are deadly. In fact, excessive pathogenicity is an evolutionarily poor strategy for a virus' survival unless balanced with a number of other trade-offs such as transmission rate and recovery rate [36] . Most parasitic infections occur horizontally (though there are rare exceptions [37] ) and the equilibrium between pathogenicity and other trade-offs is crucial to ensuring continuation of the viral lineage. While parasitic viruses by definition have some level of pathogenic effect on their host, it is not the intent of the virus to do so, because viruses don't have intentions. Viral genomes (particularly those of RNA viruses) exist as a pool of non-identical, related sequences, termed quasispecies, and are constantly adapting in response to selective pressures [38] . In some cases, the virus' environment can select for changes that create a balance of the above-mentioned trade-offs where pathogenicity may be a successful, or at least not detrimental, strategy [39] . In this light, I will define types of parasitic viruses according to both their degree of pathogenesis and the way in which they interact with their host.