Definition Computer Viruses

Software that is created specifically to help detect, prevent and remove malware (malicious software).

Antivirus is a kind of software used to prevent, scan, detect and delete viruses from a computer. Once installed, most antivirus software runs automatically in the background to provide real-time protection against virus attacks.

Comprehensive virus protection programs help protect your files and hardware from malware such as worms, Trojan horses and spyware, and may also offer additional protection such as customizable firewalls and website blocking.

Types of Computer Viruses

Viruses can find their way onto just about any device – from Windows PCs and Macs to iPhones and Androids. 

That’s right. Even phones are susceptible, particularly Android phones –  a recent report found that Android devices were responsible for 26% of all infected devices, including Windows PCs, IoT devices and iPhones.

And while Macs are often said to be more secure than PCs, latest data shows that this is actually not true, with more threats being now detected per endpoint on a Mac compared to a Windows computer. 

But either way, don’t worry. We’ve summed up the most common computer viruses and how you can avoid them below:

Macro Virus 

This type of computer virus is normally found in Microsoft Office programs. Word and Excel files are favorite places for macro viruses to embed themselves. Like most other viruses (except space fillers – more on these later) macro viruses increase the size of files when they infect them, as they attach their own code. Once a macro virus infects a file, it can easily spread to other computers when that file is shared, for example via email.

Boot Sector Virus

Much like beepers and "Tamagotchis" boot sector viruses are terrible little things that were big in the 90s. One of the oldest types of viruses, boot sector viruses go straight for the core of your computer, affecting the startup or ‘boot’ process. Back in the day, these viruses were spread through floppy disks. Nowadays, they attach themselves to emails or USB sticks. If your computer catches one of these, you’ll need an expert to carry out a full system reformat (Windows, Mac). 

Trojan Horses

Taking their name from the huge wooden horse in which the Greeks hid to get inside the ancient city of Troy, these are among the sneakiest of computer viruses. Used by cyber-criminals, Trojan horses are disguised as normal programs, tempting you to install them on your computer. Once installed, the viruses gain access to your computer's files and capture your private data – think passwords and online banking information. This can then be used by hackers to make online purchases with your bank account or expose your private information.

Overwrite Virus

When it comes to pests, these take first prize. Overwrite viruses typically take over a file and wipe the original code without you even knowing it. Once deleted, the original files cannot be recovered and the data is lost. They often spread through emails via attachments or through file downloads on the internet, so always scan files with an anti-virus software before downloading and opening them. This will help you detect these viruses before they even have a chance to cause problems.  

Browser Hijacker

The digital equivalent of a crooked cold-caller salesman, browser hijackers take over your internet searches and redirect you to pages you didn’t even want to visit. While not as harmful as other types of viruses – they trick you rather than steal from you – browser hijackers are still an annoying problem since they significantly lower your UX when surfing the internet. If you experience lots of random pop-up adverts or your usual homepage browser has changed without you telling it to, chances are you’ve got a browser hijacker. Always scan email attachments before opening and avoid downloading files from suspicious websites to avoid browser hijackers.

Web Scripting Virus

This is one for the modern age. Web scripting viruses are very clever little bugs that blend into the background of popular websites – usually social media platforms. They disguise themselves as normal links, tempting you to click on them. Then, like a greedy schoolyard bully, they steal your cookies. Ok, different cookies – we’re talking the kind of cookies that store your information. Still, web scripting viruses definitely shouldn’t be taken lightly – they can send spam to your system and damage your data, and can spread faster than most viruses.   

Polymorphic Virus


This is one sly virus. Like a chameleon changing its color for every situation, a polymorphic virus modifies whenever it replicates. This makes it hard for most anti-virus programs to keep up. Once it’s found its way onto your computer – usually through an email attachment or a download from a suspicious website – it has free reign to delete your files, steal your data, and generally sabotage your system. The bad news is these shape-shifting charlatans are on the rise. The good news is that the top antivirus programs are starting to adapt alongside them, using more sophisticated scanning techniques to catch these chameleons in the act.  

Resident Virus

One of the most common types of virus you’ll (hopefully not) come across, these sneaky little squatters find their way into your computer’s memory, completely uninvited, and make themselves at home. Just think of your old flat mate’s annoying friend who would invite them self over every night and steal your Ben and Jerry’s from the fridge. Resident viruses can come from email attachments, infected downloads or shared files and are then activated whenever your computer performs a specific action and can even attach themselves to anti-virus software, scamming the very thing that’s tasked with bringing it down. The best solution? Get an expert involved.

Multipartite Virus

Sometimes called ‘multi-part viruses’ these flexible fiends are on the rise. While most viruses either attack a computer’s central boot sector or through its files, a multipartite can do both. A versatile virus is hard to prevent, which explains the rapid growth rate of multipartite in recent years. They usually spread through .exe files – so programs like Word and Excel. How do you know if your computer is infected with a multipartite virus? Well, these viruses eat up your virtual memory like nothing else, so expect lots of ‘your computer has low virtual memory’ messages and a sudden slowing down of your computer.

Space Filler Virus 

Also known as ‘‘cavity viruses’, space filler viruses find the empty spaces in a program code and climb right in by adding their own code. This way, they don’t alter the size of files (as other viruses do), which makes them very hard to detect. Most antivirus programs find it hard to catch space fillers, but there are a few manual virus-hunting tools that tech experts use to root them out. Thankfully, they’re not only hard to detect, but they’re also hard to create and there aren’t many file types that they can attach to. This makes space filler viruses very rare. 

Direct Action

A direct action virus accesses a computer’s main memory and infects all programs, files, and folders located in the autoexec.bat path, before deleting itself. This virus typically alters the performance of a system but is capable of destroying all data on the computer’s hard disk and any USB device attached to it. Direct action viruses can be avoided through the use of antivirus scanners. They are easy to detect, as is restoring infected files.

File Infector 

A file infector is one of the most common computer viruses. It overwrites files when they are opened and can quickly spread across systems and networks. It largely affects files with .exe or .com extensions. The best way to avoid file infector viruses is to only download official software and deploy an antivirus solution.