Viruses, worms and Trojan Horses are all malicious programs that can cause damage to your computer,
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.