Computer viruses are malicious software programs that spread from one computer to another and interfere with computer operation. Viruses can enter computers through normal online activities like sharing files, visiting infected websites, or opening email attachments. Once installed, viruses may damage programs, delete files, reformat hard drives, replicate endlessly, or flood networks with traffic, causing computer crashes and disruption. Some of the most notorious viruses of the past include Melissa, Anna Kournikova, My Doom, Sasser, and the Storm Worm.