This document summarizes 5 of the worst computer viruses of all time: Melissa, ILoveYou, Code Red, Sasser, and Conficker. Melissa and ILoveYou spread worldwide in the late 1990s via email attachments. Code Red targeted Microsoft web servers in 2001. Sasser in 2004 exploited a Windows vulnerability to slow and crash systems, affecting over 1 million computers. Conficker/Downadup emerged in 2008 and turned infected systems into botnets, sometimes demanding ransom.