What is a Computer Virus? The 5 Types You Need to Know

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Computer Virus

A computer virus is not that different from a real-life virus like, say, the flu. When contracted, they can both remain dormant in the system only to become active when you least expect them to. If you detect the threat early and take appropriate measures on time, you will be able to keep the symptoms under control and recover very quickly. Left untreated, however, not only can viruses spread to others but they can also wreak havoc on the health and wellbeing of their host.

Key takeaway: A computer virus is malicious software or code that infects files and programs on your computer. Opening an infected file will execute the virus code and result in damage to your files, your computer, and your overall internet security. Read on to learn about the most common types of computer viruses.

Tip: Don’t become infected. Invest in great antivirus software today.

What is a Computer Virus? Types & Examples

What is a Computer Virus?

A computer virus is executable malicious software or code that self-replicates by taking control of other programs on an infected computer. Designed to spread from one host computer to another, a computer virus latches onto a piece of software or a document and remains there until a user opens the file in question. When they do, the virus will start executing its code and cause damage to the host computer.

There are many ways how you can contract a computer virus – via file downloads, email attachments, compromised software installations, or scam links on social media. You can then spread the virus by sharing infected files or links with others. If your computer is part of a network, just one user opening a compromised file on their desktop could be enough to take the entire network down.

Although there have been some “good” viruses with helpful effects on host machines, computer viruses are bad by definition. When executed, they can spam your email and social media contacts, corrupt files on your hard drive, and slow down your computer. Viruses can steal your passwords and change your login data to lock you out of your email and social media profiles, online banking accounts, or even your computer. In the worst-case scenario, they can wipe all the data from your hard drive in just seconds.

Computer Virus Types

There are well over a million viruses in the world, with many more created daily. They are evolving very fast, so those that were once considered extremely dangerous are now routinely taken care of by the best antivirus software. Based on their severity and the way in which they can affect your machine, there are about a dozen types of computer viruses. Here are the five worst types that you should know.

1. Macro Viruses

Perhaps the most common type of computer viruses, macro viruses attach themselves to files made in programs that support macros, sequences of commands that can be executed with a simple keystroke. These viruses are most commonly found in Microsoft Word documents and Excel spreadsheets.

Most often distributed via email attachments, macro viruses are activated when you open the infected file. If you do it directly in your email client, the virus will then send the exact copy of the file to all addresses in your contacts list. If you download the file to your computer and then open it, the macro virus will spread to other .docx and .xls files on your computer network and alter their content.

2. File Infectors

Whereas macro viruses usually infect files made in Microsoft Office, file infectors attach themselves to executable files with .exe and .com extensions. When you open an infected file to launch a program, you will unknowingly launch the virus, too. The virus can then overtake the program and spread to other executable files on your hard drive or your computer network.

The main purpose of file infectors is to compromise files and data on users’ machines and networks, create peer-to-peer botnets, and disable security software on connected computers. Some file infectors will rewrite all executable files that launch on startup, thus effectively taking control of the computer. There have also been cases where macro viruses had completely reformatted infected hard drives.

3. Browser Hijackers

As their name suggests, browser hijackers take control over certain features in your web browser. They usually change your homepage to some fake search engine and overwrite the settings so that you can’t change it. When you type an address and hit Enter, the virus will take you to a completely different website and ask you to click on a banner or sign up for something to access the site you want to see.

In most cases, browser hijackers are intended to generate revenue for their developers by showing clickable ads within the browser. They are most often bundled with free software and browser toolbars that offer advanced search features. Thankfully, they are easily detected by most antivirus programs.

4. Web Scripting Viruses

Web scripting viruses target popular websites, usually in a very sneaky way. These viruses overwrite the code of the website to insert links and videos that will install malicious software on users’ computers. In many cases, website owners don’t even know that they’re hosting potentially harmful content. All an experienced hacker has to do to infect a page is to write the malicious code and post it as a comment.

Some web scripting viruses do little more than serve you textual and visual ads to generate income for their creators. However, some can steal your cookies and use the information to post on your behalf on the infected website. Luckily, most antivirus programs will alert you when visiting a malicious website.

5. Boot Sector Viruses

Although not as prevalent now as they once were, boot sector viruses can still appear in one form or the other. Back when computers were booted from floppy disks, these viruses were very common. They would infect the system partition of the hard drive and launch on computer startup.

Nowadays, these viruses are mostly distributed through surviving forms of physical media – USB and external hard drives. They no longer pose a major threat, though, as most operating systems have safeguards that protect the boot sector of the hard drive from malicious software. Even if they do somehow infect your systems, most antivirus programs can remove boot sector viruses with ease.

Computer Virus Examples

Some of the most dangerous and/or most notorious examples of computer viruses include the following:

  • Melissa was a macro virus that spread via infected email attachments and caused $80 million in damages. Its founder David L. Smith served a 20-month prison sentence and was forbidden from accessing computer networks without authorization.
  • Yankee Doodle was a Bulgarian-made non-destructive file infector that would start playing the song “Yankee Doodle” on infected computers every day at 5 PM.
  • Shamoon is a destructive virus that could wipe all data from a network of computers in a matter of seconds. Developed as a weapon in cyberwarfare against the Saudi energy sector, it was isolated in 2014, only to make a return (as “Shamoon 2”) two years later.
  • Klez was a macro virus that would disable antivirus software on the infected computer and spam the victim’s email inbox to prevent them from receiving new messages.

How to Remove a Computer Virus

There are many programs, both free and paid, that promise to keep your computer safe from threats, but only the best antivirus software delivers on the promise. These programs will scan your system for threats and inform you whenever they detect one. Depending on the severity of the virus, they will either quarantine the infected file or remove it completely to prevent the virus from multiplying.

Some viruses are so destructive that they can wipe all your data and render your hard drive unusable. When they strike, it might be too late to do anything, which is why prevention is important.

With reliable software installed on your computer, no virus will be able to infect your documents and files. To ensure optimal protection, make sure to always use the latest version of the software and to download virus definition updates daily. The best antivirus software (like Norton,  BitDefender, Intego or Panda) will do all of this automatically, allowing you to surf freely without worrying about viruses and other threats to your online safety.


Tibor Moes

Tibor Moes

Founder of SoftwareLab

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