A computer virus is a type of malicious software (malware) designed to replicate itself and spread from one computer to another, often without the user's knowledge. It attaches itself to legitimate programs or files and activates when these are run or opened. Once active, a virus can corrupt or delete data, steal sensitive information, disrupt system operations, or use the infected computer to spread to others. Viruses can be transmitted through email attachments, downloads, infected websites, or removable media. Protecting against computer viruses involves using up-to-date antivirus software, practicing safe browsing habits, and regularly updating operating systems and applications to patch vulnerabilities.