I found this very fine introductory note on the net, courtesy www.answers.com.
A Virus is a Software used to infect a computer. After the virus code is written, it is buried within an existing program. Once that program is executed, the virus code is activated and attaches copies of itself to other programs in the system. Infected programs copy the virus to other programs.
The effect of the virus may be a simple prank that pops up a message on screen out of the blue, or it may destroy programs and data right away or on a certain date. It can lay dormant and do its damage once a year. For example, the Michelangelo virus contaminates the machine on Michelangelo’s birthday.<!– D(["mb","
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Viruses Must Be Run to Do Damage
\nA virus is not inserted into data. It is a self-contained program or code that\nattaches itself to an existing application in a manner that causes it to be\nexecuted when the application is run. Macro viruses, although hidden within\ndocuments (data), are similar. It is in the execution of the macro that the\ndamage is done.
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E-Mail Attachments Are Suspect
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\nFile attachments in e-mail messages are a common way of infecting a computer,\nproviding the user clicks on the attachment. If the attachment is an executable\nfile, it can do anything when run. Examples of executables are files with\nextensions such as .BAT, .COM, .EXE, .SCR, and .SHS
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Viruses Are Relatively Recent
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\nThe term virus was coined in the early 1980s, supposedly after a graduate\nstudent presented the concept of a program that could “infect” other\nprograms. Since then, more than 80,000 viruses have been defined. However, 99%\nof the infections are from only a few hundred variants found “in the\nwild.”
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Be Careful Out There!
\nIf you use the Internet for any purpose, be sure you have an antivirus program\nrunning at all times.
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On-lyneTech 