Nimda is a computer worm that was first discovered in September 2001. It spreads through various methods, including email attachments, network shares, and web servers. Once it infects a system, it can replicate itself and cause significant damage by deleting files and creating backdoors for other malicious software. The name "Nimda" is derived from the word "admin" spelled backward, highlighting its ability to exploit administrative privileges. It affected both Windows and Linux operating systems, making it a widespread threat. Due to its rapid propagation, Nimda became one of the most notorious worms of its time.