Where does virus collide?

The submicroscopic infectious agent that can copy itself only inside the living cells of a host, built from genetic material packaged in a protein coat and, in some cases, a lipid envelope. Beijerinck's 1898 Über ein Contagium vivum fluidum named the cause of tobacco mosaic disease a contagious living fluid that passed through filters too fine to hold back any known bacterium, introducing the idea of an agent smaller and simpler than a microbe. Stanley's 1935 work showed that this agent could be isolated as a crystallisable protein, fixing the modern picture of the virus as a particle. The referent is a biological entity that infects cells and cannot reproduce on its own, and it is not a program that copies itself inside a computer or a model for how a message spreads between people.[1][2]

Unit

A single virus particle, or virion, made of a genome plus a protein coat

Recovery means

Not applicable

Measured by

Counted as infectious titre by plaque or focus assays, and by particle counts under electron microscopy or by genome copies

A piece of software that copies its own code into other programs or files, so that it spreads whenever those hosts are run, often carrying instructions that damage or disrupt the systems it reaches. Cohen's 1987 Computer viruses: Theory and experiments gave the first formal definition and proof, describing a program that modifies other programs to include a possibly altered copy of itself, and showing by construction that such spread is possible on ordinary computers. The referent is an abstract self-replicating routine defined by what it does to other code, and it is not a living particle that infects cells or a term for the person-to-person spread of a marketing message.[3]

Unit

One self-replicating program or code fragment that inserts copies of itself into other files

Recovery means

Not applicable

Measured by

Tracked by infection rate and spread across files or machines, and detected by signature matching or behaviour analysis

A model of how a product, message, or idea spreads through a population by person-to-person transmission, so that each adopter passes it to others the way an infection moves through a host population. Rayport's 1996 article The virus of marketing put the metaphor to work, arguing that marketers should build messages that are caught and passed along rather than broadcast. Leskovec, Adamic, and Huberman's 2007 The dynamics of viral marketing measured real recommendation cascades across millions of people and modelled how far and how effectively they actually travel. The referent is a pattern of social diffusion among people, studied through networks and adoption curves, and it is not a biological infective particle or a self-copying computer program.[4][5]

Unit

One diffusion process in which adopters pass a message on to further adopters

Recovery means

Not applicable

Measured by

Quantified by reproduction or transmission rate, cascade size, and adoption curves across a network