ISAv is an orthomyxovirus (related to influenza viruses) that is thought to have evolved in Atlantic salmon; the first incidence of disease was reported among salmon farms in Norway in 1984.  The virus is extremely infectious and can result in high mortalities (in excess of 70%).  The main route of infection is “horizontal” (fish-to-fish), though it is also suspected that ISAv can be passed “vertically” (through the eggs) because it was introduced into Chile, where no adult salmon were imported or previously existed.  ISAv may also infect herring, cod, Pollock, sablefish, and other marine species.  These species, which do not typically show signs of the infection, serve as a “reservoir” that makes eliminating the virus, once present, impossible because they can shed virus that will re-infect salmon.