Nonlytic Egress and Transmission in the Virus World

Viruses must egress from the cells in which they have replicated to spread and propagate. Historically, viruses have been classified into enveloped and nonenveloped forms: Enveloped viruses exploit cellular membrane-trafficking pathways to egress while maintaining cell integrity, and nonenveloped viruses, i.e., those lacking a membrane around their capsids, lytically egress. Here, we make the compelling case that all animal and plant and many archaeal and bacterial viruses egress through nonlytic pathways. Most of these nonlytic pathways can be separated into those that enable viruses to spread without leaving the confines of cell bodies and those that traffic them to the extracellular space in enveloped membrane-bound forms. Nonlytic egress pathways bestow viruses with distinct transmission advantages including high multiplicity of infection, quality control over transmitting infectious units, and evasion of innate and adaptive antiviral immune defense mechanisms.