A virus is code written with the malicious intent of causing destruction to other people's
computers.
Unlike the programs that are on your Windows
desktop which you execute by double-clicking with your mouse, a virus is executed usually in
secret and triggered by another program. These other programs are called infected files.
The virus could actually replace itself as the executable file of another program. Destruction could
be as bad as totally wiping out the contents of your hard disk drive or overwriting your PC's system BIOS
or a combination of both, rendering your computer totally useless!
The usual sources of computer virus contamination are:
- attached programs via email;
- using somebody else's contaminated software (usually through USB drives or floppy disks);
- a security hole in your network.
The most common sources are the first two, the third has also become quite popular nowadays with the explosion of high speed internet access by way of DSL or cable modems.
TROJAN
This is an allusion to Greek mythology where Greek soldiers hid inside the hollow belly of a
wooden horse which was offered as a peace offering to the Trojan army after they had soundly routed
the Greeks.
As the Trojans were drunkenly celebrating their victory over the Greeks, the Greek
soldiers came out of the wooden horse (which was now in Trojan territory) and slaughtered all the
Trojan citizens.
A trojan virus works in the same way--it's an intruder that lies hidden somewhere in
your PC's hard drive and does its damage without the owner even noticing. A trojan can be triggered or it
can run itself, causing loss of data and may also steal important data stored on your hard drive (identity
theft).
WORM
A virus requires a contaminated program to execute; so not unlike a trojan a worm can actually
propagate itself.
Worms usually affect networked environments, such as offices, internet providers, web hosts etc.
Worms are usually embedded in other files, such as Microsoft Word documents. Once the document is read, the worm
is released and begins to travel from network to network, carrying the infected document's macro with it.
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