Computer worms are programs that reproduce, execute independently and travel across the network connections. The key difference between a virus and worm is the manner in which it reproduces and spreads. A virus is dependent upon the host file or boot sector, and the transfer of files between computers to spread, whereas a computer worm can execute completely independently and spread on its own accord through network connections.

The security threat from worms is equivalent to that of viruses. Computer worms are skilled of doing an entire series of damage such as destroying crucial files in your system, slowing it down to a large degree, or even causing some critical programs to stop working. Two very prominent examples of worms are the MS-Blaster and Sasser worms.

The original computer worm was (perhaps accidentally) unleashed on the Internet by Robert Tappan Morris in 1988. The Internet Worm used sendmail, fingerd, and rsh/rexec to spread itself across the Internet.

The SQL Slammer Worm founded in 2003 used vulnerability in Microsoft SQL Server 2000 to spread itself across the Internet. The Blaster Worm also founded in 2003 used vulnerability in Microsoft DCOM RPC to spread itself.

The Melissa worm founded in 1999, the Sobig worms founded in 2003 and the Mydoom worm founded in 2004 all spread through e-mail. These worms shared some features of a Trojan Horse, in that they spread by tempting a user to open an infected e-mail attachment.

Mydoom also attempted to spread itself through the peer-to-peer file sharing application called KaZaA. The Mydoom worms attempted a Denial of Service (DoS) attack against SCO and Microsoft.

Computer worms which spread through vulnerabilities in network services can best be protected against by keeping the antivirus up-to-date and installing patches provided by operating system and application vendors. This includes worms like SQL Slammer and Blaster.