editorial Welcome to the virus social Long-known to happen in other realms of the microscopic and macroscopic worlds, social interactions in viruses are increasingly being appreciated and have the potential to infuence many processes, including viral pathogenesis, resistance to antiviral immunity, establishment of persistence and even life cycle choice. ngoing efforts to characterise show that the ability of a vesicular stomatitis communication system to determine the virosphere have identified virus to suppress interferon (IFN)-mediated the number of recent infections in the Oviruses in every environment innate immunity is a social altruistic trait population by measuring the level of a studied, infecting every life form and even that, though costly for the viruses that phage-encoded peptide, and switch to a parasitizing other viruses. In addition to carry it and produce less progeny in the lysogenic lifestyle to prevent killing off their this vast viral diversity, variants frequently short term, enables the replication of other host when the amount of peptide increases emerge within populations of a given virus members of the viral population that do not over a certain threshold5. Cooperation also through mutation, deletion, recombination repress IFN (ref. 2). The demonstration that allows phage populations to resist bacterial or reassortment. Co-circulation of different social evolution rules govern viral innate CRISPR-mediated immune defence; initial viruses in the same areas of the world, immune evasion and virulence provides phage resistance may not be sufficient to sharing hosts and vectors, increases the a framework for future study of viral overcome the immune response, but creates chances of co-infection and co-transmission social traits. A vibrant, self-denominated an immunosuppressive state that enables — both of which are known to occur, for sociovirology community is coalescing subsequent infection by other phages in example, with Chikungunya, Zika and around the study of these traits3 and has the population6,7. Given the ecological Dengue virus. Virions were traditionally organised what promises to be an interesting importance of bacteriophages in a wide thought to transmit as individual entities, session on ‘The Social Lives of Viruses’ at variety of ecosystems, from soil and marine but we now know that groups can be the American Society for Microbiology environments to the mammalian gut, it will transmitted in vivo as one infectious annual meeting later this month. be interesting to see whether and how virus unit, as aggregates of enveloped or non- The interactions between viruses and interactions affect their roles in processes enveloped viruses, within exosomes or their defective viral genomes (DVGs) are such as biogeochemical cycling and plasma-membrane-derived vesicles, or by also important determinants of infection horizontal gene transfer. concentrating on the surface of mammalian outcomes, an issue that Marco Vignuzzi Co-infections of viruses and other gut bacteria1. Indeed, genetically diverse and Carolina Lopez discuss in a Review microorganisms have also been shown virions have been shown to initiate published online later this month in to lead to interactions that have major infection in a single cell. Some virus–virus Nature Microbiology (in press). DVGs modulatory effects on pathogenesis and host interactions have been well characterised, have been described in most RNA virus response, as explored in two recent Nature such as superinfection exclusion, whereby populations, and have been associated with Microbiology articles studying influenza– a primary viral infection induces resistance immune stimulation, viral attenuation and Streptococcus pneumoniae interactions8,9. to subsequent infections by similar viruses; persistence. Recent advances have shed With growing evidence that microbial reassortment, which is for example an light on the mechanistic basis of DVG interactions are widespread and functionally important driver of changes in influenza function, including their interference important, we are happy to have tickets to virus host range; or between replication- with full-length virus replication and the this social! ❐ defective viruses — such as adeno-associated structural characteristics that make them viruses or hepatitis D virus — with the potent triggers of the innate immune helper viruses on which they depend. response. Single cell analyses have shown Published online: 22 May 2019 However, despite being widespread in that paramyxovirus DVGs and full-length https://doi.org/10.1038/s41564-019-0486-6 nature, we know surprisingly little about the viruses preferentially infect two different extent of viral interactions that could alter cell populations and lead to different References transmission or the course of infection. infection outcomes, which is important 1. Aguilera, E. R. & Pfeifer, J. K. Virus Res. 265, 43–46 (2019). 2. Domingo-Calap, P. et al. Nat. Microbiol. https://doi.org/10.1038/ Social interactions were initially to promote viral persistence, highlighting s41564-019-0379-8 (2019). described and used to understand animal the need to consider both the single 3. Díaz-Muñoz, S. L., Sanjuán, R. & West, S. Cell Host Microbe 22, behaviour, and were soon studied in cell and population-level effects of 437–441 (2017). bacterial and unicellular eukaryote DVG–virus interactions4. 4. Xu, J. et al. Nat. Commun. 8, 799 (2017). 5. Erez, Z. et al. Nature 541, 488–493 (2017). populations. Perhaps because viruses Viruses can interact with those 6. Borges, A. L. et al. Cell 174, 917–925 (2018). were considered too simple to interact co-infecting the same host, but also 7. Landsberger, M. et al. Cell 174, 908–916 (2018). socially, this concept has only recently been communicate with those that will 8. David et al. Nat. Microbiol. https://doi.org/10.1038/s41564-019- 0447-0 (2019). adopted in virology. In this issue of Nature infect subsequently. Some temperate 9. Rowe et al. Nat. Microbiol. https://doi.org/10.1038/s41564-019- Microbiology, Rafael Sanjuan and colleagues bacteriophages use the arbitrium 0443-4 (2019). NATURE MICROBIOLOGY | VOL 4 | JUNE 2019 | 905 | www.nature.com/naturemicrobiology 905.