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Written By
*TechInfo Tech
Date Created
Tue, May. 6 2008
Updated By
Heather Shalhoub
Date Modified
Wed, Jun. 17 2009
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   Computer Virus Information


What is a computer virus and how does it spread?


Introductory Information

Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation. A computer virus is designed to spread itself by first infecting executable files or the system areas of hard and floppy disks and then making copies of itself.  Viruses usually operate without the knowledge or desire of the computer user.

How do computer viruses propagate?

by Tom Kelly, Center for Learning and Technology

There is no possible way to explain all the various means used to send viruses via the web, but here is an example that might give you an idea why you may receive them and, why, sometimes, you are wrongly accused of sending a virus. No matter what precautions are taken here at the college level or on your personal PC at home, there is always a chance that a virus may get through any anti-virus software. This is because in most cases, the software developers react to the discovery of a virus after it has already infected PC's and the (virus) software company then writes software (updates) to identify and clean or delete it.


Mary is thinking about applying to Empire State College. Her first step is to go to our web site to see what we offer. In doing so, she views many pages, most of which have a contact e-mail address listed on the page. (Every time you visit a new web page, a copy of that page is stored on your local computer in a "cache" directory until a limit is reached at which time the oldest cached pages are discarded.) Mary decides to send an e-mail to Financial Aid, which, in turn, will generate a response from someone in that office.

Now, Mary receives an e-mail message from a friend of hers that has an attachment that contains a virus. She opens the attachment and, in most cases, nothing appears to happen to her PC. However, the virus has been executed on her PC and installs itself. Each time the PC is turned on, the virus is activated and it looks for a way to send itself to other users. Typically it will check the PC address book (Outlook e.g.) and it will also search the "cached" files for e-mail addresses. It finds e-mail addresses from the "cached" version of our web pages, the e-mail for the person in Financial Aid who responded to Mary's request, etc., etc. If it is one of the newer viruses (since mid- 2001) it has its own program for mailing e-mails. Therefore, it can send out the e-mail to anyone it wishes. It may make up a name or use one from "Cache" or the address book. In turn, the e-mail sent by Mary's PC to Financial Aid and the person in Financial Aid will probably not have Mary's e-mail address, but a fictitious one that may belong to someone else.

There are many other ways that viruses are disseminated, but from the example above you can see how there is very little chance to determine the source. Until everyone has anti-virus software installed on their PC's and updates it regularly, viruses will continue to propagate.


DEFINITIONS (extracted from http://www.f-prot.com/glossary/index.html)

Boot sector virus
A boot sector virus usually spreads via infected floppy disks. When a user unintentionally leaves the infected disk with a boot sector virus, the boot sector of the user’s local drive (C:\) will also be infected. Boot sector viruses simply take up memory space or may contain a malicious load. The simplest method to avoid Boot sector viruses is to alter the CMOS settings to boot from the local C:\ drive first, rather than from floppy.

Hoax warnings are typically scare alerts started by malicious people and passed on by innocent users who think they are helping the community by spreading the warning. If you receive a warning about a security threat, please look into it further before you forward it to other users.

Internet worm
Unlike a virus, an Internet worm does not infect other files. It creates copies of itself over and over again until it exhausts system resources. The best known Internet worm was "Loveletter", which was actually a mixed threat: a mass-mailing Internet worm, an overwriting file virus, and a password-stealing Trojan.

Macro virus
Basically, macro viruses reside in documents from languages like Visual Basic, Microsoft Office and most commonly Word and Excel. Just like any other viruses, loss of critical data is the common effect.

Software that includes any threatening programs that are meant to be destructive, such as viruses and worms.

Spam is unsolicited junk e-mail. From the sender's point of view, it is a form of bulk mail. To the receiver, it is usually considered to be junk e-mail. It's roughly equivalent to unsolicited telephone marketing calls except that the user pays for part of the message since everyone shares the cost of maintaining the Internet. Spammers typically send an e-mail to a distribution list with millions of addresses, expecting only a tiny number of readers to respond to their offer. Spam has become a major problem for all Internet users.

Stealth virus
A virus that hides itself in the boot sector so it remains undetected, making it difficult to disinfect. It has the ability to alter data to hide the virus by intercepting the boot sector.

A Trojan is a program made to appear harmless in order for users to be duped into installing it, or is installed without their knowledge.

A virus is a piece of programming code, usually disguised as something else, that causes some unexpected and usually undesirable event. A virus is often designed so that it is automatically spreads to other computer users. Viruses can be transmitted as attachments to e-mail messages, as downloads, or be present on a diskette or CD. The person from whom the e-mails appear to be sent, or from whom the downloaded files or diskettes originate, is often unaware of the virus. Some viruses wreak their effect as soon as their code is executed; other viruses lie dormant until circumstances cause their code to be executed by the computer. Some viruses are playful in intent and effect ("Happy Birthday, Ludwig!") and some can be quite harmful, erasing data or causing your hard disk to require reformatting.

Worms are malicious programs that copy themselves from system to system, rather than infiltrating legitimate files. Worms are often mass mailers

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