Co-infection is an infection of more than one pathogen. In an aquatic environment, the most common occurrence is the appearance of infectious pancreatic necrosis virus (IPNV) in the presence of other viruses such as infectious hematopoietic necrosis virus (IHNV), viral hemorrhagic septicemia virus (VHSV), infectious salmon anaemia virus (ISAV), or salmonid alphavirus (SAV). In most cases, the IPN virus reduces the proliferation of other viruses in cell cultures or in the internal organs of salmonids; for example, in IHNV or ISAV co-infections. However, it also happens that there is no significant effect on the multiplication of the virus with which it coexists, e.g. IPNV-VHSV. A body's defense mechanisms, interferon and other interferon-like factors or mutations in the genome play an important role in co-infection.