A computer virus is a type of malicious software designed to replicate itself and spread from one computer to another. It can attach itself to legitimate programs or files, and when these are opened or executed, the virus activates and can cause harm to the system. This harm can include deleting files, stealing information, or slowing down the computer's performance. To protect against computer viruses, users can install antivirus software, which scans for and removes these threats. Regularly updating software and avoiding suspicious downloads or email attachments can also help prevent infections.