Self-replicating: yes
Population growth: positive
Parasitic: yes
A virus is malware that, when executed, tries to replicate itself into other executable code; when it succeeds, the code is said to be infected? The infected code, when run, can infect new code in turn. This self-replication into existing executable code is the key defining characteristic of a virus. When faced with more than one virus to describe, a rather silly problem arises. There's no agreement on the plural form of "virus." The two leading contenders are "viruses" and "virii;" the latter form is often used by virus writers themselves, but it's rare to see this used in the security community, who prefer "viruses." If viruses sound like something straight out of science fiction, there's a reason for that. They are. The early history of viruses is admittedly fairly murky, but the first mention of a computer virus is in science fiction in the early 1970s, with Gregory Benford's The Scarred Man in 1970, and David Gerrold's When Harlie Was One in 1972. Both stories also mention a program which acts to counter the virus, so this is the first mention of anti-virus software as well. The earliest real academic research on viruses was done by Fred Cohen in 1983, with the "virus" name coined by Len Adleman. Cohen is sometimes called the "father of computer viruses," but it turns out that there were viruses written prior to his work. Rich Skrenta's Elk Cloner was circulating in 1982, and Joe Dellinger's viruses were developed between 1981-1983; all of these were for the Apple II platform. Some sources mention a 1980 glitch in Arpanet as the first virus, but this was just a case of legitimate code acting badly; the only thing being propagated was data in network packets. Gregory Benford's viruses were not limited to his science fiction stories; he wrote and released nonmalicious viruses in 1969 at what is now the Lawrence Livermore National Laboratory, as well as in the early Arpanet. Some computer games have featured self-replicating programs attacking one another in a controlled environment. Core War appeared in 1984, where programs written in a simple assembly language called Redcode fought one another; a combatant was assumed to be destroyed if its program counter pointed to an invalid Redcode instruction. Programs in Core War existed only in a virtual machine, but this was not the case for an earlier game, Darwin. Darwin was played in 1961, where a program could hunt and destroy another combatant in a non-virtual environment using a well-defined interface. In terms of strategy, successful combatants in these games were hard-to-find, innovative, and adaptive, qualities that can be used by computer viruses too. Traditionally, viruses can propagate within a single computer, or may travel from one computer to another using human-transported media, like a floppy disk, CD-ROM, DVD-ROM, or USB flash drive. In other words, viruses don't propagate via computer networks; networks are the domain of worms instead. However, the label "virus" has been applied to malware that would traditionally be considered a worm, and the term has been diluted in common usage to refer to any sort of self-replicating malware. Viruses can be caught in various stages of self-replication. A germ is the original form of a virus, prior to any replication. A virus which fails to replicate is called an intended. This may occur as a result of bugs in the virus, or encountering an unexpected version of an operating system. A virus can be dormant, where it is present but not yet infecting anything - for example, a Windows virus can reside on a Unix-based file server and have no effect there, but can be exported to Windows machines.
