On May 3, 1983, computer scientist Fred Cohen presented a formal analysis of self-replicating programs that is widely cited as one of the first systematic treatments of what later became known as computer viruses. Cohen’s work grew out of experiments and theoretical study begun in 1981–1982 while he was a graduate student at the University of Southern California. He framed replication in computational and security terms, demonstrating that a program could reproduce itself by attaching to other programs and discussing conditions under which replication could be detected or prevented. Cohen’s 1983 paper, often titled “Computer Viruses — Theory and Experiments” in later references, combined laboratory experiments with mathematical and conceptual analysis. He implemented demonstration programs on Unix systems and explored mechanisms of propagation, the role of operating system features, and basic defensive strategies. The work deliberately treated self-replicating code as a subject for rigorous study rather than mere anecdote, seeking to clarify definitions, threat models, and the theoretical limits of detection. Context and antecedents Cohen’s analysis did not emerge in a vacuum. Ideas about self-reproducing automata trace back to theoretical work by John von Neumann in the 1940s, who investigated machine self-replication in principle. In computing culture, examples and rumors of self-replicating or parasitic programs circulated in the 1970s and early 1980s, including benign experiments and system utilities with propagation-like behavior. Cohen’s contribution was to bring formal experiment and security-oriented analysis to the subject and to publicize the practical feasibility of program replication within contemporary operating environments. Content and implications Cohen defined a computer virus in operational terms—programs that can infect other programs by modifying them to include a possibly evolved copy of the original code—and showed how such programs could spread in multi-user systems. He analyzed detection challenges, noting that simple signatures could be evaded and that certain theoretical limits would complicate perfect detection. He also discussed countermeasures such as access controls, checksums, and other integrity mechanisms, and emphasized a defensive, research-driven approach. Reception and legacy Cohen’s work quickly became influential in both academic and practitioner communities. It helped shift discourse from anecdote to analysis and stimulated subsequent research into malware detection, system integrity, and computer security as a field. The term “computer virus” was already in informal use in some circles, but Cohen’s formalization and demonstrations reinforced the label and framed the problem as a pressing security concern. Caveats and attribution While Cohen’s 1983 paper is frequently cited as the first large-scale conceptual treatment, it is part of a longer intellectual lineage. Earlier theoretical work on self-reproducing systems (von Neumann) and informal or experimental instances in computing communities predate it. Different scholars and practitioners may emphasize different milestones—experimental demonstrations, publicized incidents, or theoretical foundations—when tracing the origins of computer viruses. Cohen’s work, however, stands out for combining hands-on experiments with formal analysis and for catalyzing broader attention to the phenomenon. Why the date matters Dating the first comprehensive concept to May 3, 1983, highlights when the idea received systematic, security-focused treatment rather than isolated experiments or theoretical speculation. That framing helps explain why subsequent years saw rapid development of both offensive malware techniques and defensive research: researchers and system administrators now had a clearer conceptual model of how self-replicating code could behave and spread in real computing environments.