Computer viruses have pervaded popular culture at least as successfully as they have the world's computer population. Much of their popular presence is attributable to an obvious but deep biological analogy: computer viruses replicate by attaching themselves to a host (a program or computer instead of a biological cell) and co-opting the host's resources to make copies of themselves. Symptoms can range from unpleasant to fatal. Computer viruses spread from program to program and computer to computer, much as biological viruses spread within individuals and among individual members of a society. There are other computer pathogens, such as the "worms" that occasionally afflict networks and the "Trojan horses" that put a deceptively friendly face on malicious programs, but viruses are the most common computer ill by far.
Computer viruses can trace their pedigree to John von Neumann's studies of self-replicating mathematical automata in the 1940s. Although the idea of programs that could infect computers dates to the 1970s, the first well-documented case of a computer virus spreading "in the wild" occurred in October 1987, when a code snippet known as the "Brain" virus appeared on several dozen diskettes at the University of Delaware. Today viruses afflict at least a million computers every year. Users spend several hundred million dollars annually on antivirus products and services, and this figure is growing rapidly. Most viruses attack personal computers (PCs). More than 10,000 viruses have appeared so far, and unscrupulous programmers generate roughly another six every day. Fortunately, only a handful have been detected far afield.
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