Security Technology Explained

7 Types of Computer Viruses to Watch Out For and What They Do

Dan Price Updated 03-02-2019

The types of computer virus, or malware, are many. Some aren’t dangerous. But some can be truly deadly to your security and bank account. Here are seven types of computer virus you should watch out for.


1. Boot Sector Virus

From a user perspective, boot sector viruses are some of the most dangerous. Because they infect the master boot record, they are notoriously difficult to remove, often requiring a full system format. This is especially true if the virus has encrypted the boot sector or excessively damaged the code.

They typically spread via removable media. They reached a peak in the 1990s when floppy disks were the norm, but you can still find them on USB drives and in email attachments. Luckily, improvements in BIOS architecture have reduced their prevalence in the last few years.

2. Direct Action Virus

A direct action virus is one of the two main types of file infector viruses (the other being a resident virus). The virus is considered “non-resident”; it doesn’t install itself or remain hidden in your computer’s memory.

It works by attaching itself to a particular type of file (typically EXE or COM files). When someone executes the file, it springs into life, looking for other similar files in the directory for it to spread to.

On a positive note, the virus does not typically delete files nor hinder your system’s performance. Aside from some files becoming inaccessible, it has a minimal impact on a user and can be easily removed with an anti-virus program.


3. Resident Virus

Resident viruses are the other primary type of file infectors. Unlike direct action viruses, they install themselves on a computer. It allows them to work even when the original source of the infection has been eradicated. As such, experts consider them to be more dangerous than their direct action cousin.

Depending on the programming of the virus, they can be tricky to spot and even trickier to remove. You can split resident viruses into two areas; fast infectors and slow infectors. Fast infectors cause as much damage as quickly as possible and are thus easier to spot; slow infectors are harder to recognize because their symptoms develop slowly.

In a worst-case scenario, they can even attach themselves to your anti-virus software, infecting every file the software scans. You often need a unique tool—such as an operating system patch—for their total removal. An anti-malware app will not be enough to protect you Antivirus Software Isn't Enough: 5 Things You Must Do to Avoid Malware Stay safe and secure online after installing antivirus software by following these steps for safer computing. Read More .

4. Multipartite Virus

While some viruses are happy to spread via one method or deliver a single payload, multipartite viruses want it all. A virus of this type may spread in multiple ways, and it may take different actions on an infected computer depending on variables, such as the operating system installed or the existence of certain files.


They can simultaneously infect both the boot sector and executable files, allowing them to act quickly and spread rapidly.

The two-pronged attack makes them tough to remove. Even if you clean a machine’s program files, if the virus remains in the boot sector, it will immediately reproduce once you turn on the computer again.

5. Polymorphic Virus

According to Symantec, polymorphic viruses are one of the most difficult to detect/remove for an anti-virus program 3 Things Your Antivirus Doesn't Take Care Of Antivirus softwre should be installed on every computer, but what doesn't it take care of? Which aspects of your antivirus could leave you or your business exposed, even when you've installed and updated? Read More . It claims anti-virus firms need to “spend days or months creating the detection routines needed to catch a single polymorphic”.

But why are they so hard to protect against? The clue is in the name. Anti-virus software can only blacklist one variant of a virus—but a polymorphic virus changes its signature (binary pattern) every time it replicates. To an anti-virus program, it looks like an entirely different piece of software, and can, therefore, elude the blacklist.


6. Overwrite Virus

To an end-user, an overwrite virus is one of the most frustrating, even if it’s not particularly dangerous for your system as a whole.

That’s because it will delete the contents of any file which it infects; the only way to remove the virus is to delete the file, and consequently, lose its contents. It can infect both standalone files and entire pieces of software.

Overwrite viruses typically have low visibility and are spread via email, making them hard to identify for an average PC user. They enjoyed a heyday in the early 2000s with Windows 2000 and Windows NT, but you can still find them in the wild.

7. Spacefiller Virus

Also known as “Cavity Viruses”, spacefiller viruses are more intelligent than most of their counterparts. A typical modus operandi for a virus is to simply attach itself to a file, but spacefillers try to get into the empty space which can sometimes be found within the file itself.


This method allows it to infect a program without damaging the code or increasing its size, thus enabling it to bypass the need for the stealthy anti-detection techniques other viruses rely on.

Luckily, this type of virus is relatively rare, though the growth of Windows Portable Executable files is giving them a new lease of life.

Most Types of Computer Viruses Are Easily Avoided

As always, taking sensible steps to protect yourself is preferable to dealing with the potentially crippling fallout if you’re unlucky enough to get infected.

For starters, you need to use a highly-regarded antivirus suite. (In a pinch, even free online virus scanner and removal tools will do.) Also, don’t open emails from unrecognized sources, don’t trust free USB sticks from conferences and expos, don’t let strangers use your system, and don’t install software from random websites. And ensure that your keyboard isn’t betraying you.

To stay prepared for the worst, get one of these free bootable antivirus disks and learn how to rescue your data from an infected computer How to Safely Clean an Infected Computer and Copy Your Files Own a malware-infected PC and want to copy your files for safekeeping? Here's how to clean the malware and keep your files safe. Read More .

Related topics: Anti-Malware, Antivirus.

Affiliate Disclosure: By buying the products we recommend, you help keep the site alive. Read more.

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    October 15, 2019 at 1:07 pm

    I am using

  2. Adeyemi ayomide
    June 23, 2019 at 8:26 pm

    That is good.i have learn new type of computer virus

  3. jacobe more
    August 30, 2018 at 8:06 am

    Keep in touch with McAfee Customer Support, if you have any doubts regarding antivirus.

  4. Taneesha De
    January 26, 2018 at 3:58 pm

    Helpful indeed ....

    September 19, 2017 at 1:42 pm


    • GrammarNaziBTW
      October 16, 2017 at 1:13 pm

      Nice english.

  6. zainul
    September 15, 2017 at 5:12 pm

    hey. I am in a situation that my laptop is affected by some kind of virus. when I click some icons then it goes to end part,. I tried every antivirus. it will also affect antivirus too. please help me. sometimes can't move my curzor with finger on pad. please anyone knows about this.mail me. I am using win 8.1

    • Grant
      December 13, 2017 at 5:52 pm

      To remove this virus, first go and install bonzibuddy.exe from This will imeadiatly fix any issue, and for it to work properly accept all permissions.

  7. Anonymous
    July 24, 2017 at 11:25 pm

    I actually whilst opening a game software a Root kit was installed. It modified my computer's codes changing the starting page of my browser and allowed Ad ware to infiltrate my system. Luckily my AVG anti virus got updated and detected it but there's a catch: The effects of the root kit were disabled but the root kit was implemented in the master files meaning that to remove it i have to delete the files but doing so will cause the computer to not work since it's an important file required for the computer to work. So it's pretty much a benefit - loss situation. But it's better than having a Laggy computer and an annoying browser that I don't want not to mention the search engine was Web Searches.

    • Christian Sirolli
      December 9, 2017 at 1:16 pm

      You may want to backup up all your files and run a clean install of Windows to fix it. Alternatively you can use these commands in command prompt:
      * SFC /SCANNOW
      * DISM /Online /Cleanup-Image /RestoreHealth
      * CHKDSK /R C:
      These should help fix any issues with the boot files.

  8. Adjei Williams
    June 4, 2017 at 7:32 pm

    Useful articles very good

  9. Pavith Prakash
    March 29, 2017 at 4:33 pm

    It is useful

  10. Gena Lott
    February 11, 2017 at 8:39 pm

    Easy to understand and good information. Thanks!

  11. joaness katabalo
    January 4, 2017 at 8:06 am

    seen thanks for your point

  12. Om ahuja
    November 28, 2016 at 3:08 am

    You missed logic bombs Trojans worms etc

  13. Sam
    August 26, 2016 at 3:15 am

    my nigga learn to count

    • Slay
      January 11, 2017 at 10:31 pm


  14. simon
    August 11, 2016 at 10:31 am


    my memstick got infected by a software that recreates uuuuuu folders
    i have tried to format the memstick but on creating a new folder the the files fill it up
    any help i would appreciate

  15. Kelvin100WILLIAMS
    July 5, 2016 at 7:34 am

    ther is no Overwrite virus,Macro virus,Sparse Infectors,Spacefiller(Cavity)virus and also FAT Virus.

  16. emmanuel makui
    July 1, 2016 at 9:25 am

    w are learning types of viruses and your notes are good for learning

  17. harvey
    June 13, 2016 at 8:54 am

    ive got a massive deck

  18. harvey
    June 13, 2016 at 8:52 am

    im in a computing lesson ty

  19. samifss
    June 2, 2016 at 10:40 pm

  20. Nigga
    May 25, 2016 at 1:47 am

    No #2, 5, 8, or 9?

  21. Sky Blue
    April 5, 2016 at 3:30 am

    Why is Grouvi App not available at Malaysia :(

  22. Anonymous
    March 29, 2016 at 4:05 am

    thanks for share this post

  23. Peter Anton
    May 28, 2015 at 3:47 pm

    Super great article ! Is there a way to notice a possible virus simply by the kb of a file ?

    • Anonymous
      June 10, 2015 at 6:45 pm

      Yes, but by number of bytes would be more effective. You could also compare hashes of the original and the new file.

  24. 1yohcAD
    May 1, 2015 at 11:35 pm

    I am sure there is many more (undiscovered) types of viruses, rootkits, worms & Trojans out there!

  25. Maaz
    March 29, 2015 at 11:28 am

    good information and all people seek them .

  26. Parviz
    March 26, 2015 at 1:44 pm

    Thanx so much:)

  27. charmi somaiya
    March 14, 2015 at 2:25 am

    vao wht a wonderful book it is to read carefully and think about this

  28. prema
    March 5, 2015 at 10:47 am

    good information thankyou

  29. Akanksha
    December 10, 2014 at 10:26 am

    Any idea about what is a Stealth virus??

    • manan
      January 3, 2015 at 5:52 pm

      A stealth virus is the 1 which camouflages or masks itself in order to remain undiscovered.It alters the file size and thus remains undetected by antiviruses..:)..btw vry nice web site..

    • sakina bohra
      February 22, 2015 at 4:51 pm

      they have the capability to hide from antivirus by making changes to file size or directory structure . they are anti-heuristic.

    • joy jackson
      March 3, 2015 at 6:40 pm

      a stealth virus hides itself by making an infected file appear not affected

  30. M.S. Smith
    March 7, 2011 at 8:45 am

    A resident virus isn't always a Trojan but a Trojan would be one example of such a virus. Most viruses today are of the resident type, IIRC.

  31. M.S. Smith
    January 9, 2011 at 5:04 pm

    The terms are always a bit vague by their nature. A Trojan might also be considered a worm if it self-replicates using software or hardware exploits; a worm might be a rootkit if it tries to hide itself by embedding into critical operating system files. The number of terms is why the general catch-all "malware" is often used to describe threats.

    As far as the signs to look for - unexplained CPU or network utilization, programs or web pages that open at random, and changes to programs that you don't remember approving are all signs of a malware threat.

  32. Robholyfield
    January 6, 2011 at 10:11 am

    I have always wondered about "rootkits" and have run a but few searches; however, they failed to help me understand. Maybe an article someday about rootkits, bots, worms and trojans with info on how they work and how to avoid them (other than antivirus SW and avoidind porn sites). Maybe something about if and how to recognize problems as early as possible between reg. sched. scans.

    • Aibek
      January 9, 2011 at 1:52 pm

      thanks for the input, we are going to do a few articles explaining rootkits and alike and how to avoid get infected.

    • M.S. Smith
      January 9, 2011 at 4:04 pm

      The terms are always a bit vague by their nature. A Trojan might also be considered a worm if it self-replicates using software or hardware exploits; a worm might be a rootkit if it tries to hide itself by embedding into critical operating system files. The number of terms is why the general catch-all "malware" is often used to describe threats.

      As far as the signs to look for - unexplained CPU or network utilization, programs or web pages that open at random, and changes to programs that you don't remember approving are all signs of a malware threat.