Ask Leo! by Leo A. Notenboom

What's the difference between a 'Trojan Horse' a 'Worm' and a 'Virus'?

Search First! Then browse: Categories | Full Archive | By Date | Newsletter

Home » Viruses and Malware

Summary: We hear about Trojan Horses, Viruses and Worms, but what do they mean? They're loosely related types of malware with different ways to cause trouble.

What's the difference between a 'Trojan Horse' a 'Worm' and a 'Virus'?

Seems like there's no shortage of confusing terminology in the computer biz. With the advent of computer viruses over recent years, we've spawned even more terminology that often seems only to make things less clear.

And then recently it looks like we can't even spell! I mean, really ... "phishing"? What's that all about?

The good news is that it's not really that difficult. Let's run down the terms.

Virus: we've all heard this one too often lately. In a sense, "virus" is the root definition of the things we'll talk about here.

"A virus is just a computer program."

A virus in the human body is an organism that replicates (makes copied of) itself and overwhelms the body's own defenses making it sick. Human borne viruses can spread in several ways from person-to-person. Depending on the type of virus, catching it could be as simple as breathing the same air as an infected person. It might require direct contact, or it might require an even more direct transfer of, say, blood.

The term "virus" when applied to computers sounds very similar. A computer virus:

  • is a program - really, that's all any of this is. A virus is just a computer program. It's written by some individual or individuals, presumably with the intent of spreading and causing grief.

  • makes the infected computer "sick" - in the computer sense, "sick" can mean poor performance, crashes, lost files and data, or more.

  • replicates itself - just like you can copy a file from one disk to another and have copies on both disks, a computer virus is in part defined by its ability to make copies of itself. Typically the copies aren't on the infected computer, but rather on other computers, which leads us to the last characteristic...

  • infects other computers - exactly how depends on the virus, of course, but another key defining point for a computer virus is that it can spread on its own.

Worm: technically, a worm is a virus that does no direct damage to the computer it's infected. In reality, worms can cause a great deal of trouble merely by getting passed from one computer to many others, and they can clog up a network very quickly.

Unfortunately there isn't necessarily agreement on that definition. At least one other resource I've seen states that a) a Worm does cause damage to the infected system, and b) worms and viruses differ from how they are transmitted: a worm is a stand-alone program, while a virus propagates by attaching itself to another program.

Trojan Horse: a program that claims to be one thing but is, in fact, another. A trojan horse is not a virus per se but it may carry them. For example many people consider Kazaa, the music sharing software: a trojan horse because it carries with it a bunch of spyware. There are trojans that claim to be patches for a problem, often arriving in email, that are in fact spyware and virus installers.

Phishing: I think of phishing as a kind of email trojan horse. It's email that looks like it comes from some official site such as your bank, Paypal, or eBay, but in fact it comes from someone pretending to be them. They'll ask you to go visit a site, or provide some information, looking very official and proper, except that the site is not what you think and the information you give them allows them to steal your credit card or identity.

The bottom line, of course is that we all need to keep aware of these issues and act accordingly. We shouldn't have to, of course; hackers shouldn't exist and operating systems and other software should simply protect us. But the pragmatic reality of the situation is that we do need to keep our guard up.

Listen to the audio:

Or
Download the mp3

What does that mean? How should you protect yourself? As outlined in my earlier article How do I keep my computer safe on the internet? is boils down to common sense, a firewall, and running up-to-date anti-virus, anti-spyware tools regularly.

Related:

Helpful? Get new articles weekly by email in my FREE newsletter!

Your Name:
Your Email:


Why Subscribe?

Article C2201 - October 9, 2004

Was this article helpful? «Yes» «No»

Recent Comments
11 Comments

I would try one, or more, other anti-virus programs: http://ask-leo.com/recommendation_antivirus_software.html

Posted by: Leo at September 11, 2005 9:16 PM

its a good article but it wud be better if you wud have given the difference between all the three in the table format.

Posted by: dhanvi at October 7, 2007 1:35 PM

is there any other cure to trojan except reinstalling the operating system??if so,plz reply.

Posted by: Anoop at January 25, 2008 8:26 PM

My son and I use Avast protection on our home computers. Since May he has had a problem with his computer and the issue is the following aspimgr.exe

aspimgr.exe has slowed his computer and deleted his System Restore Points.

Is there any help for him, or do you have any suggestion for him to repair. He has discovered that an Error window with the following message:

The instruction at "0x00919d04" referenced memory at "0x000ab000" the memory could not be "read"

This problem started around the end of May and we discovered tonight if he closes the error it will lock his computer up. To night we discovered that if he does not close the error it seems to work.

It also gives a warning that the virus protection is out of date.

Help Please,

Thank you,

Robert Grahn

This note was written for Andy Grahn

Posted by: R. Grahn at July 22, 2008 11:34 AM

You have done a good job by explaining the difference between a virus, a worm and a Trojan Horse. Keep up doing the goodjob.

Posted by: Lungelwa at August 13, 2008 5:57 AM

Post a comment on "What's the difference between a 'Trojan Horse' a 'Worm' and a 'Virus'?":






(Email Address will not be published.)

Remember Me?

By popular demand...
my tip jar
Cuppa Joe
Buy Leo a Latte!

(you may use HTML tags for style)

RSS feed Subscribe to the RSS Feed specifically for comments on this article.

Before commenting, please...

  • Read the article at the top of this page. If your comment shows you didn't, it'll be deleted and ignored.

  • Comment only on this article. Use the Google search box at the top of the page if you have a question about something else.

  • Don't include personal information in the comment. No email addresses. No phone numbers. No physical addresses.

  • Don't spam. Excessive links to unrelated sites within a comment or across multiple comments will cause all such comments to be removed.

  • Don't ask me to recover lost passwords or hacked accounts. I can't, and those comments will be deleted.

  • I can't respond to every comment. And I can't vouch for the accuracy of others who do.

Please wait. Your comment is being processed ...


Question? Ask Leo!