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What Is The Difference Between A Worm, A Trojan & A Virus? [MakeUseOf Explains]

Chris Hoffman 12-03-2012

difference between worm and virusSome people call any type of malicious software a “computer virus,” but that isn’t accurate. Viruses, worms, and trojans are different types of malicious software with different behaviors. In particular, they spread themselves in very different ways.


Malicious software in general is referred to as “malware.” If you want a catch-all term for bad computer software, malware is the word to use. The good news is that your antivirus program doesn’t care what the malicious software is called — antivirus programs also remove worms, trojans, and other types of malware.

Viruses Infect Other Files

A computer virus infects other files, similar to how a biological virus infects living cells. When you execute a virus by running an already-infected file, the virus infects other files on your system. In most cases, it adds itself to existing .exe files on your system, so it will run when they do.

Some types of viruses 7 Types of Computer Viruses to Watch Out For and What They Do Many types of computer viruses can steal or destroy your data. Here are some of the most common viruses and what they do. Read More can also infect other types of files, such as macros in Word or Excel documents. The virus spreads when an infected file is transmitted to another system. This may happen via an email attachment, removable storage device or a network file share.

difference between worm and virus

Viruses also wreak havoc on your system. In some cases, they may replace existing program files with themselves entirely instead of just adding themselves to the existing programs. They may delete files and announce their presence. Viruses can also take up system memory and cause crashes.


Viruses are dangerous because of how they spread. When a legitimate file moves between systems, the virus comes with it.

Worms Copy Themselves

A worm is a standalone program that doesn’t require user intervention to spread. Worms don’t infect existing files – they spread copies of themselves instead.

Some worms, like the infamous Mydoom worm A Brief History Of The 5 Worst Computer Viruses Of All Time The word "virus" and its association with computers was affixed by American computer scientist Frederick Cohen who used it to describe "a program that can 'infect' other programs by modifying them to include a possibly... Read More , email copies of themselves to every address in a computer’s address book. Some of the most dangerous and fast spreading worms, such as the Blaster and Sasser worms, exploit vulnerabilities in network services. Instead of emailing files, they travel over the network and infect unpatched systems that aren’t running firewalls.

difference between worm and trojan horse


Worms that spread over the network can generate a large amount of traffic, slowing down the network. Once a worm is on your system, it can perform the same malicious actions that a virus can.

Trojans Lie In Wait

Trojans are named after the mythological trojan horse. To conquer the city of Troy, the Greeks constructed a wooden horse and presented it to the Trojans as a gift. The Trojans accepted the gift into their city. Later that night, Greek soldiers emerged from the inside of the hollow wooden horse, opened the city gate – and you can imagine what followed.

A trojan horse is the same sort of thing. Trojan horses masquerade as useful software, such as a legitimate program or an illicit crack for a legitimate program. Instead of being well-behaved software, a trojan opens a backdoor on your system.

difference between worm and trojan horse


The trojan’s author can use the backdoor to make your system part of a botnet Top Five Computer Crimes & How to Protect Yourself From Them Read More , use your Internet connection to perform illicit activities that will be traced back to you, download other malware programs onto your system, or do anything else they want.

Trojans don’t try to spread themselves in any way. Trojans must be manually executed by a user.

Other Threats

These aren’t the only types of malware. Here are a few other common ones:

  • Spyware – Spyware spies on you. Spyware encompasses everything from “key loggers” that log your keystrokes to steal your credit card information and online banking passwords, to advertising programs that monitor your web browsing activity and send it over the Internet. Spyware is generally designed to make money for its creators. Some antivirus programs don’t detect and remove this sort of software, but Windows Defender 3 Ways Windows Defender Can Protect Your PC Read More , built into Windows 7, does.

difference between worm and virus.

  • Scareware – Scareware, also known as crimeware, often appears as a fake antivirus alert on a web page. If you believe the  alert and download the fake antivirus program, it will inform you that you have viruses on your system. The antivirus program asks for a credit card number, insisting on a payment before “fixing” your system. Scareware holds your system hostage until you pay or remove it Use Norton Power Eraser To Aggressively Scan & Clean Scareware Read More .

Keep your operating system and other software up-to-date and run an antivirus program The 10 Best Free Antivirus Software No matter what computer you're using, you need antivirus protection. Here are the best free antivirus tools you can use. Read More to protect yourself from these dangers.

Is there anything else you want to know about viruses, worms, trojans, or other type of malware? Leave a comment and we’ll answer any questions you have.

Image Credit: Infected Computer via Shutterstock, Bacteriophage via Shutterstock, Worm in Email via Shutterstock, Trojan Horse Photo from Alaskan Dude, Hand Reaching Out From Laptop via Shutterstock

Related topics: Anti-Malware, Spyware, Trojan Horse.

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  1. pat
    March 3, 2016 at 11:49 am

    Beware FileZilla is a Trojan

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    • qqq1qq
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  3. Faisal Ahmed
    August 25, 2012 at 5:25 am

    Tech-jargon's explained with example in simple way...easy to catch and commit to memory.

  4. radhika
    August 7, 2012 at 3:28 pm

    can u plz give me a detail explanation abt trojan plzzzzzzz

  5. shaurya boogie
    June 23, 2012 at 7:54 pm

    I never knew about scareware. thnx...

  6. Ben
    June 20, 2012 at 9:51 am

    I usually have a tough time explaining the difference between viruses, worms and trojans to highly inquisitive clients. This article takes this load off my shoulders. I will be referring inquisitive clients here!

    • Chris Hoffman
      June 20, 2012 at 11:00 pm

      Awesome! Glad we could provide such a useful resource!

  7. kelly
    June 11, 2012 at 11:56 pm

    Very interesting, I thhink I have some malware now. Plus I no why its so important to remove all other programs before adding another antivirus. Thank you very very helpful

  8. Revind
    March 22, 2012 at 6:23 am

    (Paperback) I've been in the computer cntusloing field for over 10 years (DOS, Novell, and Microsoft), and during all that time I've been fighting viruses, and now emailing worms, for my clients. I've always installed the latest antivirus software and told my clients how to treat suspicious emails or files. And while this advice has always worked on my company's network, it seems rarely to have worked at my customer's locations. Mr. Grimes talks about the same experiences in the book. (I was passed an early copy of the book by one of my friends in the antivirus industry). To make a long story short, the book's advice works. It's all commonsense stuff after you read it, and it showed me some new prevention tactics that I will continue to use with existing and future clients.As for example, one of my clients, with about 100 workstations, seemed to get infected about every three days no matter what I would tell them (one person in particular). And although I love the cntusloing dollars, it really became a pain disinfecting their network again and again. I followed the steps in the book, and my clients haven't been infected since. It's only been a month, but they went from dozens of infections per week, to none.Every chapter in the book covers a particular topic, like Windows viruses, Instant Messaging attacks, Email attacks, etc. Mr. Grimes begins by describing the underlying technology, talks about specific attacks, and then tells how to detect and prevent them. Each chapter has dozens of recommendations and his last chapter (actually second to last) talks about what steps you should take on each PC you supervise. This was nice because trying to remember the dozens of steps to take all at once would have been tough. He even covers how to make an anti-virus plan, but that doesn't really apply to my cntusloing work; however I'm sure it would help a company system admin type.I can easily say I learned more about Virus attacks on Windows, Java, VBA, and Internet apps than I knew before. I was really surprised by how many places bad programs can hide to automatically start on a computer. There are over a dozen. And for a technical book it was really easy reading. It was the best book I've ever read, and used, on computer viruses. The book included his email address and I sent a question to him and he answered it the same day. The book covers Windows, no Linux or Mac, but doesn't cover Novell Groupwise in the email chapter but it is well worth the read. Although most of my clients are Microsoft shops, I've still have some Novell shops. It mentions, but doesn't discuss automated distribution tools, like SMS or ZenWorks. If you've got a very large network, you're going to need a good way to automate all the steps. There are few typos and grammar errors, but certainly not enough to take away from the message. Also, like most other antivirus books, it doesn't say what antivirus product to get. Mr. Grimes mentions a few different products, but I was really looking for his recommendation (or anyone) to tell me the best antivirus product out there. It seems they all miss something. Other than these few issues, great book and I highly recommend it.

    • Sam
      July 31, 2012 at 7:45 am

      Mr. Grimes who? What book?

      • Chris Hoffman
        August 1, 2012 at 10:05 am

        That's a good question.

    March 20, 2012 at 2:36 am

    these are just using the source code of they doesnt even come close.

  10. Emmanual
    March 17, 2012 at 10:55 am

    Good articles here!

    • Chris Hoffman
      March 17, 2012 at 6:17 pm

      Thanks for the compliment! Enjoy the site!

      • puja more
        August 2, 2012 at 1:21 pm

        yup rytttt nice1

  11. Emmanual
    March 17, 2012 at 10:53 am

    Good ideas for IT techn like me I knew thinfs there. Thank!

  12. Gerry
    March 16, 2012 at 3:38 am

    Nice article. I usually lump all of these things into the Malware category. You didn't mention "phishing" but then again maybe you consider that a tactic. There are also the "promiseware" pitches that promise to eliminate all of these problems and "double your speed" which feed off the lack of knowledge and information you have presented.

    • Chris Hoffman
      March 16, 2012 at 3:49 am

      Yup, phishing is a tactic. And I generally just call everything "malware" too.

      I've never heard the phrase "promiseware," although I'm familiar with that snake oil type of software. Thanks for expanding my vocabulary!

  13. Ankur
    March 13, 2012 at 2:07 pm

    Good post. Something every computer user should know.

    • Chris Hoffman
      March 13, 2012 at 10:42 pm

      Definitely! Thanks for the compliment.

      Anything else you think every computer user should know? Could make for a good post idea!

  14. Virtual Office
    March 13, 2012 at 3:06 am

    Really nice info.
    Specially about Trojan. I never knew detail about Trojans. :)

    • Chris Hoffman
      March 13, 2012 at 4:19 am

      Glad I could help! Trojans are easy to remember, given the origin of the word.

  15. Mike DeGeorge
    March 13, 2012 at 12:38 am

    I always love articles like this, because i usually forget things that i've learned. It's a good refresher. Kind of like reinstalling a program because you're missing some files. :) And either way, just reading about these things is just plain fascinating. 

    • Chris Hoffman
      March 13, 2012 at 4:17 am

      Thanks; I'm glad you enjoyed it! I enjoy explaining these things.

  16. Lee
    March 13, 2012 at 12:24 am

    Nice article! I'm pretty tech savvy but never really thought about the differences before. I'll be able to be technically-correct now when talking about these topics.

    • Chris Hoffman
      March 13, 2012 at 4:18 am

      It's helpful to understand what other people are talking about when they use these words, too!