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Worms and Viruses and Other Things, Oh My!
We call all types of malicious, unwanted software malware. Malware comes in different shapes, sizes and types. Malware includes viruses, trojans, root kits, and worms. There are subcategories, and sometime the type of malware for a given program is unclear because it has attributes of several types. So what's the difference between a worm and a virus?
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What's a Virus?
A computer virus spreads from computer to computer through the actions of users. Being exposed to the virus by downloading it, opening it in an e-mail, or loading it from removable media are the usual ways they infect your computer. It usually takes some action by a user to expose themselves to the virus. This is essentially the defining characteristic. Once the virus is loaded, it can do things on its own to infect files, stay loaded in memory and reload on restart, and so forth. When it hasn't been executed by the user or automatically by the operating system, usually it is dormant. This isn't a hard and fast rule, and viruses have many different types and subcategories, as they are complex.
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Why Is it Called a Worm?
With so many computers on the Internet malicious programs can find an enormous number of vulnerable systems. Worms take over systems, use their resources to search for others and spread. Some worms are more aggressive and spread fast, taking over hundreds or even thousands of unprotected systems very quickly. Some engage in additional destructive behavior or leave back doors in infected systems. The common attribute for worms is they automatically search for uninfected systems, infect them, and repeat. Worms can tie up system resources and they use valuable bandwidth. Some recent worms were Sobig, MyDoom and Conficker.
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What's the Difference?
You can probably see the difference from the descriptions above: worms are able to propagate and spread on their own, without any action or interaction with a user. While they may be able to do more or different things if a user interacts with them, the defining difference is that the worms spread on their own. For more information on these topics, see my articles on computer worms, how people make computer viruses, the top ten famous computer viruses, the different types of computer viruses, and zero day vulnerabilities.