Internet Worms

Worms are truly autonomous virtual viruses, spreading across the net, breaking into computers, and replicating without human assistance and usually without human knowledge. Worms are particularly interesting technological constructs, with an intriguing mathematical structure and complexity. They fascinate because they take the digital imitation of life to another step -- they autonomously search for computers, penetrate them, and replicate their intelligence to continue the process.
An Internet worm can be contained in any kind of virus, program or script. Sometimes their inventor will release them into the wild in a single copy, leaving them to replicate by themselves through a variety of stratagems and protocols.
The most successful Internet worm of all time, in terms of sheer saturation, was the code red worm, which scanned the Internet for vulnerable Windows computers running the IIS web server to install itself and continue the infection.
The first worm was released by Robert Morris, a Computer Science graduate student at Cornell University. He released it November 2, 1988. The worm eventually infected more than 6,000 computers across the Internet. He was eventually convicted and sentenced to three years of probation, 400 hours of community service, and a $10,050 fine. 

Reference:
The Internet