Virus
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
spread to other computer users. Viruses can be transmitted
as attachments to an e-mail note, as downloads, or be present
on a diskette or CD. The source of the e-mail note, downloaded
file, or diskette you've received 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.
Worm
A worm is a self-replicating virus that does not alter files
but resides in active memory and duplicates itself. Worms
use parts of an operating system that are automatic and
usually invisible to the user. It is common for worms to
be noticed only when their uncontrolled replication consumes
system resources, slowing or halting other tasks.
Trojan
In
computers, a Trojan horse is a program in which malicious
or harmful code is contained inside apparently harmless
programming or data in such a way that it can get control
and do its chosen form of damage, such as ruining the file
allocation table on your hard disk. In one celebrated case,
a Trojan horse was a program that was supposed to find and
destroy computer viruses. A Trojan horse may be widely redistributed
as part of a computer virus.
The term comes from Homer's Iliad. In the Trojan War, the
Greeks presented the citizens of Troy with a large wooden
horse in which they had secretly hidden their warriors.
During the night, the warriors emerged from the wooden horse
and overran the city.
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