Computer Virus | List, Types & Examples

 A computer virus is a type of malicious software, or malware, that infects computers and corrupts their data and software. They are always the result of human activity. Because they are designed to replicate themselves by infecting other programs on a computer, they are termed "viruses".

Computer viruses are designed to cause system disruption, significant operational issues, and data loss and leakage. They typically attach to an executable host file, resulting in the execution of their viral code when a file is opened. The code then spreads via networks, drives, file-sharing programs, or infected email attachments from the document or software to which it is attached. The way a computer virus performs is determined by the way it is coded. It could be as simple as a harmless prank, or it could be sophisticated, resulting in criminal activity and fraud. While some viruses are limited to a single device, others spread across a network environment in search of other vulnerable hosts.

There are millions of computer viruses, but the following are some of the types of common computer viruses to be aware of:

Boot Sector Virus

The boot sector virus specifically targets the boot sector of a hard drive, impairing the boot process. These viruses were once prevalent during the time when computers were booted via floppy disks. Today, these viruses spread via physical media such as external hard drives or USB flash drives.

Program Virus

When a program is installed or executed, a program virus is launched. Program viruses can spread via physical media such as a CD or via email attachments. They conceal themselves within a seemingly useful program and then execute it when the program is opened. They are frequently referred to as "trojan horse viruses" due to their ability to conceal themselves from their unsuspecting victims.

Macro VirusExamples of Computer Viruses

The following list of computer viruses are some examples of computer virus:

Worm

A computer worm is a type of trojan horse malware that, unlike traditional viruses, does not require the user's intervention to spread from device to device. After breaching a system, it can spread from one computer to another without human intervention.

ILOVEYOU

The ILOVEYOU virus is an overwrite virus. This virus disguised itself as a love letter from one of its victims' contacts and spread via email.

It was the most damaging malware event of all time when it occurred in 2000. In just about 10 days, it reached an estimated 45 million users and caused $10 billion in damages.

SQL Slammer

The SQL Slammer is a worm that attacks unpatched Microsoft SQL 2000 servers. It resulted in a denial of service on some internet hosts as well as a significant slowdown in overall internet traffic. It spread quickly, infecting the vast majority of its 75,000 victims within ten minutes.

Stuxnet

Stuxnet is a highly sophisticated computer worm that was designed to attack Iran's nuclear facilities but has since mutated and spread to other industrial and energy-producing facilities.

CryptoLocker

The CryptoLocker ransomware encrypts files on Windows computers and then demands a ransom payment in exchange for the decryption key. It first appeared on the Internet in 2013 and was aimed at computers running Windows.

Tinba

Tinba is a trojan that infects end-user devices and attempts to compromise and steal funds from their financial accounts. The Trojan was discovered in 2012 and initially infected thousands of Turkish computers.

Welchia

The Welchia worm, also known as Nachi, is a Nematode that deletes Blaster and fixes the vulnerabilities that allow them to exist. While it has no malicious intent, it does tend to slow down computers and networks. On August 18th, 2003, it was first reported.

Shlayer

Shlayer is a trojan-type virus that spreads adware and potentially unwanted applications and promotes bogus search engines.

Famous Computer Viruses

CryptoLocker, ILOVEYOU, MyDoom, WannaCry, Klez, Anna Kournikova, Sobig, and Zeus are just a few of the famous computer viruses that have infected users' computers since 2000.

As previously stated, ILOVEYOU is a dangerous computer virus that infected millions of Windows computers within hours of its release and continues to be regarded as one of the most dangerous worms ever released.

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To unlock this lesson you must be a Study.com Member.Lesson Summary

A computer virus is a type of malicious software, or malware, that infects computers and corrupts their data and software.

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Computer Viruses

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 are pieces of malicious computer coding that can be spread from one device to another. They didn't necessarily start out with the intent to damage a computer or steal data, though. 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.

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.

Common Viruses

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, Program, Macro

Boot Sector

The boot sector viruses can overtake your computer when it is booting up. They 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.

Program Virus

A program virus is launched when a program is installed or executed. They 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,'' as they are hidden from their unsuspecting victim.

Macro

The macro virus disguises itself as coding language in word processing and spreadsheets. It comes along in things like word processing and spreadsheet programs and is 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.

Hijacker, Direct Action, Resident

Hijacker

If you've ever logged on to the internet and noticed that your homepage has 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 are 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, attaches to .exe and .com files and becomes active once those files are launched. This 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.

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 it's named: fast infectors wreak havoc quickly while slow infectors take a while to do their damage.