Computer millions of dollars. There are thousands of

Computer viruses have evolved from mildly annoying to seriously destructive. In this lesson, you’ll learn more about some different types of viruses and how they function in the computer environment.

I Love You

What could be better than receiving an email with the subject line, ”I love you?”’ Well, if you were the recipient of such an email in the early 2000s, you might not have been so happy. That’s because cybercriminals used the phrase to trick computer users into opening what turned out to be a computer virus.The ”ILOVEYOU” virus as it came to be known wrote over users’ system and personal files, rendering infected computers essentially useless. At the time, the ”ILOVEYOU” virus became so widespread that it was one of the most destructive computer viruses to date.

Computer viruses, pieces of malicious computer coding that can be spread from one device to another, didn’t necessarily start out with the intent to damage a computer or steal data. Early computer viruses were born more out of curiosity than criminal intent. Today, they are big business for the people behind them – giving them access to confidential data and costing individuals and companies who fall victim millions of dollars.

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There are thousands of varieties of computer viruses so it’s not possible to create an inclusive list of every virus. But, let’s put together a list of the most common types of viruses and how they function.

Be on the Lookout

Viruses, just like viruses that can enter the human body, take on all sorts of shapes and sizes. Here are some common ones.

Boot Sector

The boot sector viruses are not quite as pervasive as they once were (when computers operated more with insertable disks), but they can still be transmitted primarily through removable media such as a USB or a flash drive.

The boot sector virus is so named because it can overtake your computer when it is booting up.

Program Virus

A program virus can also be attached to CDs, removable media or even email, but instead of infecting a computer at start-up, they go to work when you open a program, even a seemingly innocent one. These are sometimes referred to as ”trojan horse viruses,” and are hidden from their unsuspecting victim and launched when a program is installed or executed.


The macro virus comes along in things like word processing and spreadsheet programs, with the virus language written in the same macro language those programs use for their legitimate processes.

This virus, embedded in a Microsoft Word document, for example, will cause the program involved to perform a series of unintended actions immediately when the program opens.

Web Browser Hijacker

If you’ve ever logged on to the internet and noticed that your homepage has been changed or settings in your browser window have been modified without your permission, you’ve likely been the victim of a browser hijacker. Often, these viruses come in the form of adware or spyware that attach to your computer when you download some type of program from the internet.

Direct Action

One of the most common types of computer viruses, the direct action virus, installs to a computer’s memory and can infect any number of files – from those near where it’s located in the computer to files on an entire computer network at a business. After you execute a file, the virus springs to life. These viruses attach to .

exe and .com files and once those files are launched, the virus becomes active.

Resident Virus

A resident virus makes its home on your computer and can infect any file on your system, even without executing a program. It comes to life when your computer’s operating system loads, making it one of the hardest to track and remove.

In essence, a resident virus can function all on its own. These come in two common types, fast and slow infectors, each operating as its named: fast infectors wreak havoc quickly while slow infectors take a while to do their damage.

Cavity Virus

A cavity virus, sometimes called a spacefiller virus, is inserted into the empty spaces that exist in regular computer files.

Cybercriminals use this method to make a virus less detectable since it doesn’t change the size of the file, and they use it to destroy or steal sensitive information. Spacefiller viruses don’t cause infection to the program itself, but to other files on a computer.


What about a virus that makes itself hard to find by changing its name every time the infected file is executed? That’s what a polymorphic virus does, and that’s what makes it particularly hard for antivirus software to catch. Because the virus changes its name frequently, it can get around antivirus software that recognizes it as a new file.

Overwrite Virus

If the thought of losing an important file gives you nightmares, the overwrite virus is a scary one. If the overwrite virus, typically spread via email, infects one of your files, deleting the file – and losing its contents – is the only course of action.

This can be a costly experience for end users. This virus can be challenging for most people to spot.


Of all the viruses out there, this one might be the wild card. Multipartite viruses can spread in numerous ways and infect your computer based on different variables including the start-up of an operating system, software that is executed or individual files when they are opened.

Lesson Summary

Computer viruses, once mildly annoying and born out of technological curiosity, have grown to be big problems that can cause a lot of frustration over lost or stolen data and expense. Viruses can be transmitted through the internet, removable media such as flash drives or even via email.

Each virus does something slightly different. The boot sector takes over a computer upon start-up. Program viruses are hidden in programs and go to work once executed. A macro virus disguises itself as coding language in word processing and spreadsheets, while a browser hijacker attaches to web downloads and alters your browser.

Other viruses make their home on your computer (resident), overwrite your files, hide in empty files spaces (cavity virus) or change their name to avoid detection (polymorphic). The multipartite combines some of the characteristics of each, as it spreads in numerous ways and can infect a computer based on different variables.


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