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What is a Computer Virus?

by Cyberguy | Last Updated | June 22, 2022
Cyber Dictionary|CyberSecurity - Consumer|CyberSecurity - SMB

Most people have heard about or have some general understanding of what a computer virus is. They are the oldest forms of cyber threat and are called viruses because they mimic the behavior of biological viruses that we see in nature.

Viruses are nasty pieces of malware (malicious computer software) that can take control of your devices (desktop, phone, tablet, smart appliance, etc.) and cause all kinds of damage. They replicate and spread thus the linkage to their biologic cousins.

Unfortunately, if you are unlucky enough to have your device catch a virus, it very likely will wreak havoc on your device’s storage, destroy files, mess with your device’s configuration, make your device unusable, and in the worst of cases cause permanent damage to your device – and that may just be the start of the nasty things it could do.

Signs your device has been infected

One of the primary differences between viruses and other forms of malware, is that they are specifically designed to spread and replicate. So not only is your device a target but every device is a target and once the virus is on one of your devices it could spread to every other device your device is somehow connected to. This is one of the truly evil things about them.

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What are the types of computer viruses out there?

As you might expect, there are numerous computer virus types, and the bad actors are a creative bunch and constantly coming up with new and more evil types. But in general, there are 6 categories of viruses that are deployed.

Boot sector virus

These viruses take control of your device’s boot drive (typically a hard drive). They overwrite or damage the boot sector or more specifically the master boot record. This is the part of the boot drive that allows your device’s operating system to load. Once compromised your device becomes non-functioning.

Resident virus

These are similar to direct action viruses, as both are types of file infector viruses. Resident viruses install themselves on devices. They are more dangerous and trickier to eradicate than direct action viruses.

Resident viruses can infect very quickly or do so slowly to be less visible. Resident viruses need to be removed with anti-malware software immediately, as the virus can potentially infect your entire anti-virus software.

Direct action virus

These viruses work by attaching to existing .exe or .com files when installed or executed. Once this occurs, the virus spreads to other existing files and can render them inaccessible. While direct action viruses are unwanted and can hinder your ability to access files on your device, they are not usually able to delete files entirely and can be removed with an anti-virus program.

Polymorphic virus

These viruses are extremely difficult to recognize because they are constantly changing shape. They actually change their software code every time they infect a new file making it difficult for antivirus/antimalware tools to identify and eradicate them. Advance tools use other methods other than looking for code signatures to identify and remove so you can get tools to help, but these viruses are nasty little devils.

Multipartite virus

These viruses have a double dose of trouble because they can infect both your device’s boot section and your files. And they do so at the same time.

They are also resilient because even if you clean up one infected component, these viruses can jump from one file to another and cause a reinfection. So, it’s a bit like “whack-a-mole” and you need a quality hammer (antivirus/antimalware) to kill them.

Macro virus

These viruses are built in macro languages like the ones found in apps like Word, Excel, PowerPoint, etc. The fact that they are hidden inside commonly used files is one of the reasons they are very effective for hackers. These files are accessed by nearly everyone every day. So hackers know people don’t always pay attention and click when they see one of these file types, whether on a file sharing service or as an attachment to an email.

These viruses are also really nasty because they often use your own contact lists to replicate themselves by sending the same email to all of your contacts without your knowledge. They use you and your device to spread the virus even further.

10 Main Types of computer virus

So How Do I Get a Computer Virus?

There are many ways that your devices could be infected with a computer virus, but generally speaking, the five most common methods are:


Email is the most common and most successful form of infecting viruses on devices. There are a number of methods bad actors use to deliver viruses using email.

The most widely used is through email attachments. If the user opens the attachment files it unleashes the virus onto the device. Attachments can be executable files (.exe, .com, .zip, .dll, .pif, .vbs, .js, .scr) or even files that support macros (excel, word, powerpoint, etc).

Another method is through an infected email body. The advent of HTML (hypertext markup language), which supports embedded objects and executable code has made this a challenge. Many email services are disabling HTML by default to prevent/mitigate this type of virus delivery.

A third is through links in the phishing email that lure the user to bad actor websites. Prompting or luring users to take actions that result in inadvertently downloading a virus or other malware.


Instant Message or SMS is another platform that hackers use to infect devices. This happens a lot on mobile devices using the standard SMS tools provided by the services providers but can also happen on SMS apps like Skype, Facebook messenger, Snapchat, and Whatsapp.

Like emails, SMS messages can contain links or attachments that, if the user clicks or accesses the attachment, will result in the downloading of the virus.

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File sharing services

File Sharing services like Dropbox, Sharepoint, One Drive, and Google Drive. all have the potential to spread viruses from one user to another. These services sync files across devices and across users and if unchecked, it is highly possible that unsuspecting users end up accessing these infected files, which in turn infect the device they are using to access such files.

Antivirus fakeware

Antivirus fakeware is another method bad actors use to infect user devices. Often, when users surf the internet, they may access an unknown or unfamiliar site that causes some type of popup to display indicating that their device is infected and that the software promoted in the popup can fix the problem.

This is a total con job but is really scareware used to intimidate users into downloading the supposed antivirus software, which in fact is the exact opposite. It is the virus software. Scare tactics and exploiting social behavior using social engineering techniques are the chief weapons of the evildoers.

Unpatched software

Unpatched software can be a wide-open hole for hackers to access and infect your devices. This is particularly true of popular apps like Microsoft Office, Adobe Reader, and SMS apps. You need to keep your apps up to date with patches. Developers often release patches to remedy security vulnerabilities and close security gaps.

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Best Strategies to Protect Yourself from a Computer Virus?

It seems pretty clear and I would say probably obvious that it is best to avoid getting a computer virus.  They are a pain and can do real damage. If you get one, it is highly likely that you will pass it on to other devices of yours, your friends, your work colleagues, etc.

So what are the best ways to prevent and avoid getting a virus?

The first thing is to use good old common sense and basic cybersecurity practices. This includes: 

Do NOT just click links your friends, colleagues or anyone else sends you. Check, double-check and then check again that the link is safe before you click.

Do NOT open any email attachment unless you 100% know what it is. Bad actors rely on human curiosity — a common social engineering technique — to spread viruses. Do not let them lure you in.

IGNORE “Your PC is infected!” warnings. DO NOT click on hysteria popups that aren’t coming from your own antivirus/antimalware software. It’s a trap!

Disable macros in Microsoft Office and other apps that offer macros. Only reenable for MS Office documents or other files that you know are safe after you have downloaded or copied the documents/files.

Top 5 Virus of all time

But even if you diligently do all the good cyber practices above and more, invest in a quality antivirus and antimalware tool if you don’t already have one. It is worth its weight in gold. Ask anyone who has experienced a virus.

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