Viruses, Trojans and Worms : The Difference & Simmilarities

Almost every computer user knows what a virus is; some of them also have heard of Trojans Horses and worms.  Many people call all kinds of malicious programs “viruses,” and other people use the terms interchangeably.  Both are incorrect; there are differences between all three types of malware.


What They Are

A virus is a self replicating program, which means that it copies itself on the host computer.  This affects the computer’s performance and stability. Virus can be transferred from one computer to another through an “infected” flash drive, disk, or other medium.  The term virus comes from the viruses which cause diseases in living beings, because both kinds work the same way.

A worm also is a self replicating computer program.  However, a worm is able to spread across a network without the user moving it from one place to another. Unlike viruses that affect the computer’s performance, a worm affects a network’s performance and stability. It “creeps” over a network, much like a living worm wriggles over the ground.

A Trojan or Trojan Horse is a hidden, non replicating malicious program that usually comes in hidden in a legitimate program.  It comes neatly packaged inside a legitimate application such as a screen saver. The term Trojan Horse comes from Virgil’s epic poem about the Trojan War.


What They Do

Viruses, Trojan Horses, and worms all adversely affect computers, but not all of them are equally destructive.

Viruses always cause system performance to suffer.  However, they don’t always corrupt system files and data.  A particularly bad virus can be fatal by causing a system to completely crash; when this happens, the virus has infiltrated important system operations.

Worms also aren’t always destructive; however, they do eat up bandwidth and adversely affect network performance.

Trojan Horses are the darlings of hackers.  They are designed to provide unauthorized access to some all the areas of the host computer; this can provide valuable information to the hacker who is using one.


From One Place To Another

A virus isn’t able to move from computer to computer on its own.  For a virus to infect a remote machine, someone has to move it to that computer from an external source.  That kind of external source may be a CD or a flash drive, or a virus may come in attached to an application that comes over a network.

Worms exploit loopholes in the security of a target computer.  Then they creep into the target machine on their own.  A worm is programmed to locate network paths or to automatically attach itself to network application messages such as email; from there, a worm moves to the next computer.

Trojans always are spread along with legitimate programs from legitimate sources.  At first look, they appear to be useful.  However, depending on the Trojan, they may carry out data theft, create a back door to give a hacker access to your computer, spy, or install other malware (such as worm) on the host computer; that computer may then move the Trojan onto another machine.


Compare and Contrast:

  • A virus damages a system by multiplying itself; it may damage system files
  • A worm is similar to virus; however, it also creeps into other network nodes and creates problems over the network
  • A Trojan Horse allows a hacker access to the host computer and reveals private information such as passwords and account numbers

Whether a piece of malware is a Trojan, a worm, a virus or a combination of two or more of these, its presence on a computer is always damaging.