Virus vs. Trojan vs. Worm

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Difference between Virus, Trojan and Worm

Almost every computer user is aware of the terms Virus, Trojan and worms or at least the first one, the virus. In fact most people consider all types of malicious programs as ‘virus' and other use all these terms interchangeably which is technically wrong. There lie subtle differences which distinguish these malwares from one another.



A virus is a self replicating program that keeps on copying itself on the host computer and thus affecting the computer performance and stability. Virus can be communicated to other hosts only by user intervention. The term is derived from the viruses which cause diseases in living beings as both kinds work the same way.

Worm is also a self replicating computer program but it has the capability to spread across the network on its own. Unlike viruses which affect the computer performance, worms affect more of network performance and stability. Its capability to creep over the network is perhaps the reason for its name.

Trojan horse is the hidden and non replicating malicious program, which generally gets entry into the host through a legitimate program. It comes wrapped inside a legit application e.g. screen saver. The term Trojan horse comes from the Latin epic story of Trojan War.

Difference in Effects

All the malwares affect adversely the computing experience of users, though to varying degrees.

Viruses aren't always aimed to corrupt system files and data, but they always cause the system performance to suffer. Virus can also be fatal; causing a system crash by infiltrating the important system locations. Similarly, worms aren't always destructive; however they invariably consume bandwidth and affect severely the network performance. Trojans on the other hand are always intended to provide unauthorized access to certain or all the areas of the host computer which can reveal valuable information to the cracker.

Infecting Remote Machines

As stated earlier, a virus does not have the ability to infect a remote computer on a network on its own. For it to infect a remote machine, it has to be carried to that computer by some source external to it. For e.g. attaching it to an application program and then sending it over the network to the destination machine, or through CDs or flash drives etc.

Worm exploits loopholes in the security of the target computer and it creeps in to the target machine on its own. It is programmed to find out the network paths or to automatically attach itself to the network application messages such as email etc. in order to find its next victim.

Trojans are always spread along the legitimate programs from legitimate sources. They appear to be useful; however varying from Trojan to Trojan they can carry out activities such as data theft, creates a backdoor to give access of your computer to the cracker, spying, other malware (such as worms) installation on the host etc. which then may carry them to the next target machine.

Similarities and Differences

A virus damages a system by multiplying itself and by playing with the system files sometimes.

  • Worm is similar to virus but has the additional capability to creep into other network nodes and create nuisance over the network.
  • A Trojan horse allows access of the host computer to the hacker and reveals the desired information.

The bottom-line is malware whether it is Trojan, worm, virus or a blend threat (combination of these); its presence in your Computer is always injurious and it's the administrator's responsibility to tackle with it.

Which internet nuisance is most deadly to computers?
  • Virus
  • Trojan
  • Worm

Discuss It: comments 1

A firewall is a system that prevents unauthorized use and access to your computer. A firewall can be either hardware or software. Hardware firewalls provide a strong degree of protection from most forms of attack coming from the outside world and can be purchased as a stand-alone product or in broadband routers. Unfortunately, when battling viruses, worms and Trojans, a hardware firewall may be less effective than a software firewall, as it could possibly ignore embedded worms in out going e-mails and see this as regular network traffic.

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