Viruses are malicious programs that spread throughout computer files without user knowledge. Most widespread virus infections spread through email message attachments that activate when opened. The vicious cycle of a virus perpetuates as infected emails are forwarded to multiple users. Viruses also spread through shared media, such as Universal Serial Bus (USB) drives.

Initially created as pranks, viruses are responsible for widespread and significant computer system and file destruction. Installing anti-virus software helps prevent, block or remove previously installed viruses.