What is a virus and how did I get it?

WHAT IS A VIRUS?
A virus is machine code that is present on your machine that does something that your machine is not intended to do. Two examples come to mind: The "Stoned" virus which puts a message on your screen about every hundreth time you boot it up that "Your Machine is Stoned" and does nothing else and the W95.MTX virus that modifies registry entries in such a way that your system becomes unusable. These examples are on either extreme of what viruses do. Most are in the middle, some or light damage. And with probably one thousand new virii or variants being recognized each month it is just a matter of time until you are infected.
WHAT KIND OF VIRII ARE THERE?
First, a definition of what virii types are in circulation.
1) Machine code virii that exist in executable files that when activated infect other files.
2) Text file virii that execute as .bat (batch files) or .vbs (script files) to do things to the computer. Text is benign as text. It only becomes dangerous when the file is named with an extension that processes under your operating envionment.
Both of these types can then be further broken down by what they do and when they do it.
1) Destructors make portions of systems inoperative
2) Modifiers change portions of systems that eventually make systems inoperative
3) Worms replicate themselves until the storage medium is full and the system dies
And finally is the time when they do it:
1) Immediately , you know when the system stops working or gives an error message
2) Timed , happens when triggered by other events (time , day , existence of another file , etc)(these are also known as trojans, or trojan horses)
There are combinations of all of these types.
HOW DID I GET INFECTED?
Machine code infectors are generally caused by sharing of infected files. Text file infectors can be from shared files, but are more likely to be from email attachments or from script files exploiting weaknesses in email programs.

This FAQ is under construction, along with several more, for the use of Tek-Tips members and visitors.
It may contain errors , misstatements, and opinion. If you have information that should be included, or will expand its value, please forward the information to erfair@bellsouth.net so it can be included. Major contributions will be acknowledged.
This is not solely my work or ideas. Credit also goes to:
Alt255
Comtech
and others whose names or handles will be included.