A virus is a type of malware 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. This harm can include corrupting files, stealing data, or slowing down the computer's performance. Viruses often spread through infected email attachments, downloads, or removable storage devices like USB drives. To protect against viruses, users should install antivirus software, keep their systems updated, and avoid opening suspicious links or files.