A worm is similar to a virus, but does not need to attach itself to a host (such as a file, disk, or program). Worms move across networks, exploiting security holes on target computers, and then installing themselves into the system software on host machines. From its new home on a host computer, the worm searches for new targets on which to replicate, and the process continues across the Internet.

Technically speaking, worms are programs whose sole purpose is to replicate and spread themselves to other computers. Some programmers write them with no other purpose or intent than to see how far they will spread … However, in recent years, worms have been used as the vehicle by which viruses are primarily spread. Commonly, once a computer has been infected by a virus/worm (usually by opening an infected e-mail attachment), the virus component will set up and begin running an SMTP mail server, and the worm component will begin to replicate the virus/worm and e-mail it to addresses found in the computer’s e-mail address book.6


Worms may carry dangerous payload as they travel around the Internet. They may leave a Trojan horse behind or some other backdoor access to a user’s system. They may also plant a virus or damage data on target systems. One of the main indicators of the presence of a worm on your system is that performance grinds to a crawl while the machine is connected to the Internet but speeds up when it is no longer connected.

How do I protect my computer against worms?

The greatest protection against worms is to make sure that your system’s firewall is turned on. These days firewalls are standard equipment in modern operating systems, but sometimes they’re turned off. So just make sure yours is turned on. If you have questions, refer to a professional computer repair shop.

6 “Computer ‘Malware’: Worms, Trojans, Back Doors and Viruses,” Penn Computing, University of Pennsylvania (July 13, 2007). http://www.upenn.edu/computing/security/malware.php

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