Strength in numbers: Mechanisms of viral co-infection.

RNA virus populations are diverse due to a variety of factors, including lack of proofreading of the viral RNA-dependent RNA polymerase. These diverse viral populations include defective viruses incapable of productive infection. Recent studies have determined the existence of several modes of viral transmission outside of canonical pathways, including en bloc transmission of multiple viruses into a single host cell via membrane vesicles. Additionally, it has recently been determined that viral aggregation and bacteria can facilitate the delivery of multiple viruses to a single cell. Co-infection of RNA viruses is important since it has the potential to enhance viral fitness. Furthermore, through complementation and recombination, co-infection could potentially promote "resurrection" of otherwise defective viral genomes and has the potential to expand viral diversity.