A computer virus is a type of malicious software designed to replicate itself and spread from one computer to another. It attaches itself to legitimate programs or files, and when these are executed, the virus activates and can cause harm to the system, such as deleting files or stealing information. Viruses can be transmitted through various means, including email attachments, downloads, and infected USB drives. To protect against computer viruses, users can install antivirus software, keep their operating systems updated, and avoid clicking on suspicious links or downloading unknown files.