Living with the enemy: viral persistent infections from a friendly viewpoint.

Persistent infection is a situation of metastability in which the pathogen and the host coexist. A common outcome for viral infections, persistence is a widespread phenomenon through all kingdoms. With a clear benefit for the virus and/or the host at the population level, persistent infections act as modulators of the ecosystem. The origin of persistence being long time elusive, here we explore the concept of 'endogenization' of viral sequences with concomitant activation of the host immune pathways, as a main way to establish and maintain viral persistent infections. Current concepts on viral persistence mechanisms and biological role are discussed.