A computer virus is a small program that spreads from one computer to another and interferes with computer operation. Viruses can spread through email attachments, downloads, shared files, and more. Infected computers may experience slow performance, crashing, missing files, and unusual error messages. To protect computers, people should install antivirus software, update security programs, use firewalls and pop-up blockers, and backup important files. Antivirus programs detect and remove known viruses, while online scanning services can also help identify infected files.