The document defines and describes computer viruses. It explains that viruses are malicious programs that can damage systems by deleting files, displaying unwanted messages, or causing systems to crash. Viruses spread by attaching themselves to other programs or files and transferring to other computers through networks, removable drives, email attachments, and other methods. While some are created as pranks, most are deliberately designed to cause harm. Antivirus software works to detect viruses by scanning files and memory using signature detection and heuristics, and needs to be kept up to date to protect against new threats.

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