COMPUTER WORMS

(the figure above show a computer worm ^^)

A computer worm is a self-replicating computer program. It uses a network to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. Unlike a virus, it does not need to attach itself to an existing program. Worms always harm the network (if only by consuming bandwidth), whereas viruses always infect or corrupt files on a targeted computer.

Naming and history
The name 'WORM' comes from The Shockwave Rider, a science fiction novel published in 1975 by John Brunner. Researchers John F Shoch and Jon A Hupp of Xerox PARC chose the name in a paper published in 1982; The Worm Programs, Comm ACM, 25(3):172-180, 1982), and it has since been widely adopted.
The first implementation of a worm was by these same two researchers at Xerox PARC in 1978.Shoch and Hupp originally designed the worm to find idle processors on the network and assign them tasks, sharing the processing load, and so improving the 'CPU cycle use efficiency' across an entire network. They were self-limited so that they would spread no farther than intended.

Payloads
Many worms have been created which are only designed to spread, and don't attempt to alter the systems they pass through. However, as the Morris worm, and Mydoom showed, the network traffic and other unintended effects can often cause major disruption. A "payload" is code designed to do more than spread the worm - it might delete files on a host system (eg the ExploreZip worm), encrypt files in a cryptoviral extortion attack, or send documents via e-mail. A very common payload for worms is to install a backdoor in the infected computer to allow the creation of a "zombie" under control of the worm author - Sobig and Mydoom are examples which created zombies. Networks of such machines are often referred to as botnets and are very commonly used by spam senders for sending junk email or to cloak their website's address.Spammers are therefore thought to be a source of funding for the creation of such worms, and worm writers have been caught selling lists of IP addresses of infected machines.Others try to blackmail companies with threatened DoS attacks.
Backdoors, however they may be installed, can be exploited by other malware, including worms. Examples include Doomjuice, which spreads using the backdoor opened by Mydoom, and at least one instance of malware taking advantage of the rootkit backdoor installed by the Sony/BMG DRM software utilized by millions of music CDs prior to late 2005.

Worms with good intent
Beginning with the very first research into worms at Xerox PARC there have been attempts to create useful worms. The Nachi family of worms, for example, tried to download and install patches from Microsoft's website to fix vulnerabilities in the host system — by exploiting those same vulnerabilities. In practice, although this may have made these systems more secure, it generated considerable network traffic, rebooted the machine in the course of patching it, and, most importantly, did its work without the consent of the computer's owner or user.
Most security experts regard all worms as malware, whatever their payload or their writers' intentions.

Protecting against Dangerous computer worms
Worms mainly spread by exploiting vulnerabilities in operating systems, or by tricking users to assist them.
All vendors supply regular security updates-- "Patch Tuesday", and if these are installed to a machine then the majority of worms are unable to spread to it. If a vendor acknowledges a vulnerability but has yet to release a security update to patch it, a zero day exploit is possible. However, these are relatively rare.
Users need to be wary of opening unexpected email, and should not run attached files or programs, or visit web sites that are linked to such emails. However, as the ILOVEYOU worm showed, and as phishing attacks become more efficient, tricking users will always be possible.
Anti-virus and anti-spyware software are helpful, but must be kept up-to-date with new pattern files at least every few days.