but there are differences among the three.

What Is a Virus? A computer virus attaches itself to a program

or file enabling it to spread from one computer to another, leaving infections as it travels.

Like a human virus, a computer virus can range in severity: some may cause only mildly annoying

effects while others can damage your hardware, software or files. Almost all viruses are

attached to an executable file, which means the virus may exist on your computer but it

actually cannot infect your computer unless you run or open the malicious program.

It is important to note that a virus cannot be spread without a human action, (such as

running an infected program)

What is a worm? A worm is similar to a virus by design and

is considered to be a sub-class of a virus. Worms spread from computer to computer, but

unlike a virus, it has the capability to travel without any human action.

The biggest danger with a worm is its capability to replicate itself on your system, so rather

than your computer sending out a single worm, it could send out hundreds or thousands of

copies of itself, creating a huge devastating effect. One example would be for a worm to

send a copy of itself to everyone listed in your e-mail address book. Then, the worm replicates

and sends itself out to everyone listed in each of the receiver's address book, and the

manifest continues on down the line.

Due to the copying nature of a worm and its capability to travel across networks the end

result in most cases is that the worm consumes too much system memory (or network bandwidth),

causing Web servers, network servers and individual computers to stop responding.

What is a trojan horse? The Trojan Horse, at first glance will appear

to be useful software but will actually do damage once installed or run on your computer.

Those on the receiving end of a Trojan Horse are usually tricked into opening them because

they appear to be receiving legitimate software or files from a legitimate source.

Some Trojans are designed to be more annoying (like changing your desktop, adding silly

active desktop icons) or they can cause serious damage by deleting files and destroying information

on your system.

Trojans are also known to create a backdoor on your computer that gives malicious users

access to your system, possibly allowing confidential or personal information to be compromised.

Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate.