Types of Computer Viruses with Definition

Types of Computer Viruses

A computer is an electronic device that can be seen in any organization, whether it is large or small. Computers are making day-to-day tasks easier. However, computer viruses are threats that can break the security of your computer and not only corrupt your computer system files but also gain access to your confidential information. Therefore, you must know what computer viruses are and how many types of computer viruses are there. Let's start with the definition of a computer virus:

Computer Virus Definition

Computer virus is a malicious code or program designed to alter the way a computer system operates and is written in such a way that it can spread from one computer to another by itself without the user’s knowledge to operate malicious actions.

A computer virus can spread through internet downloads, e-mail attachments, the infected removable disk, social media scam links, etc. 

If a computer is infected with viruses, then there can be a variety of symptoms. Some common signs of computer virus attack are given below:


Frequent pop-up windows
Changes to your browser’s homepage
Mass emails automatically being sent from your email account
Frequent crashes in your computer
Unusually slow computer performance
Automatic start-up of unknown programs
Unusual activities such as password changes, unknown file creation, etc.

Note: The first computer virus was created by BBN Technologies between 1970–1971. This was called the 'creeper virus'. It was affecting computers by filling up the hard drive until the computer became unstable and could do no further work.

Also Read: Types of Computer Memory

Types of Computer Viruses

We have explained all the types of computer viruses that can affect your computer. Check out the computer viruses list below:


Boot Sector Virus

The boot sector virus came into existence when the floppy disk was the primary source of booting the computer. Initially, these types of viruses infected floppy disks. Later, they started infecting the first sector of the hard disk drive, where the master boot record (MBR) is stored. If a computer is infected with a boot sector virus, the virus will activate itself upon starting or booting your computer and it will take control of your computer.

This type of virus is usually spread by an infected USB drive. Removing this type of virus is a complex task and often requires formatting a computer system. Due to improvements in BIOS architecture, the risk of this virus is no longer serious.

File Deleting Virus

File deleting viruses are designed in such a way that they can automatically delete files on your computer system. This type of virus can delete system files as well as the stored data files of the computer.

Directory Virus

Directory viruses change the paths for the files. When a user runs a program that is infected with the directory virus, the virus program automatically starts running in the background along with the original program. It is very difficult to find the location of the original program or software once they are infected with this type of virus.

FAT Virus

FAT stands for “File Allocation Table”. FAT viruses affect the file allocation system of the computer’s hard drives. It is the place where the computer stores information about files and their paths.

Direct Action Virus

Direct action viruses attack on a particular type of file, typically .exe and .com files. The virus is considered 'non-resident' because it does not install itself. This virus gets activated when you execute the file containing the virus. Otherwise, the direct-action virus remains dormant on a computer. The primary aim of this kind of virus is to replicate and infect files in folders.


The direct-action virus has some limitations; it does not typically delete files or affect the performance of the computer system. Aside from some files becoming inaccessible, the virus has the least impact on a computer and can be easily removed by anti-virus programs.

Resident Virus

Unlike direct action viruses, resident viruses install themselves directing into the primary memory of your system. These types of viruses can execute anytime when an operating system loads and corrupt files and programs on your computer system. Resident viruses are hard to find, and it is even harder to remove them. CMJ, Meve, MrKlunky, and Randex are the most popular examples of resident viruses. 

Multipartite Virus

Most of the viruses are spread via one method or deliver a single payload. But multipartite viruses may infect and spread in multiple ways. These types of viruses are very infectious that they can perform different actions on an infected computer. Multipartite viruses may infect both the executable program files and the boot sector.

A two-sided attack makes these viruses difficult to remove. Even if you clean the program files of your computer, the virus remains in the boot sector. It gets activated again when the computer is restarted.

Macro Virus

Macro viruses are designed using the same macro programming languages used for MS office package or other macro-based software applications. Therefore, these viruses target applications or software that contains macros. A macro is a process of automating the repetitive tasks of any software. These macros are usually stored as part of the document or spreadsheet.

The macro viruses spread when a user opens an infected document or spreadsheet. They are usually transmitted via email attachments. The most common examples of macro viruses are O97M/Y2K, Bablas, Melissa.A, and Relax.

Polymorphic Virus

Polymorphic viruses are very difficult to detect or remove. It is because these viruses change their code each time an infected file is executed. However, the anti-virus programs usually detect the presence of any virus by detecting its code. These types of viruses are also capable of replicating easily. Some examples of polymorphic viruses are Satan Bug, Elkern, Tuareg, and Marburg.


Retrovirus

Retroviruses are different types of viruses. These types of viruses usually try to disable anti-virus software running on computer systems. There can be two ways that retroviruses may use to affect your computer; either they attack the anti-virus software and prevent it from running or they destroy the entire virus definition database.

Tunneling Virus

Tunneling viruses attempt to bypass the detection of anti-virus software by installing themselves in the interrupt handler chain. These viruses disable interception programs that run in the background of an operating system and are responsible for detecting viruses. Tunneling viruses can install themselves in device drivers.

Armored Virus

As the name speaks, the armored viruses are very difficult to detect as they are armored. It means that these types of viruses are protected from possible detections, and can remain in the computer to cause harm. Armored virus bypasses anti-virus programs by tricking that it lies somewhere else than its real location. Antiviruses wrongly focus on tricked locations, believing that it is the place where the virus is stored. Apart from it, the virus is built to make tracing, disassembling, and reserve engineering very difficult. This makes it difficult to examine its code.

Stealth Virus

Stealth viruses are so smart that they try to trick anti-virus programs by appearing like they are necessary files or programs of the operating system. Sometimes, these viruses temporarily remove themselves from the system without actual deletion so the ant-virus programs do not generally detect them. Stealth viruses are designed with anti-heuristic nature which helps them to hide from heuristic detection. Therefore, some anti-virus software cannot detect stealth viruses.

Nonresident Virus

Nonresident viruses are spread through the module. As soon as the module is executed, the nonresident virus becomes active and subsequently infects one or more files.

Types of Computer Virus

Sparse Infector

Sparse infector viruses use special techniques to reduce the chances of their detection. These types of viruses attack the computer system occasionally. Depending on their coding, they may only infect a program with every fifth or tenth execution. Because of their occasional tendency, they are hard to detect by anti-virus programs.

Overwrite Virus

Overwrite viruses can infect the content of a file. These types of viruses can infect both standalone files as well as entire pieces of software, losing the original content. The overwrite virus is usually spread through email and is hard to identify for an average user. In most cases, you have to delete the infected files to remove the virus from your computer.   

Spacefiller Virus

As its name suggests, the spacefiller viruses fill empty spaces between the codes without damaging the code or increasing its size. These viruses usually attach themselves to the file and try to alter the execution of the program or change the encrypted code. They make use of stealth techniques so the users cannot determine the increase in the file code. These viruses are also called “Cavity Viruses”. The most popular spacefiller virus is the Lehigh virus.

File Infector Virus

File infector virus is very common amongst computer viruses. This virus usually comes along with the program files, such as .com or .exe files. When a user executes an infected program file, the file infector virus gets activated. The virus does not require file an infected file every time. The infected file is used only as the initial point for file infector virus which can further re-write the file itself. These types of viruses can slow down the performance of programs and cause many other damages.

Web Scripting Virus

Web scripting viruses are attached to certain links, ads, images, videos, and the layout of a website. These viruses carry malicious codes that get activated when you click on an infected source. The virus may automatically download unwanted files on your computer or may redirect you to malicious websites.


Browser Hijacker

These types of viruses usually infect the web browser of the computer. They 'hijacks' certain functions of the browser and handle them accordingly. Typically, the homepage of your browser may change to a particular site. Or if you enter any specific domain address in the URL address bar, the browser hijacker will redirect you to multiple fake websites that may harm your computer. These types are viruses are generally used to earn revenue from web ads.

Mass Mailer Virus

Mass mailer viruses infect e-mail programs like MS Outlook and access stored e-mail addresses. Once the virus gets access to the address book, it starts sending spam or fake e-mails to the addresses saved in the address book of the particular e-mail program.

Encrypted Virus

Encrypted viruses are written and encrypted with malicious codes which make it tough for anti-viruses to detect them. These viruses can only be detected when they decrypt themselves during replication. These types of viruses usually don’t delete computer files or folders. However, they can drastically reduce the performance of the computer system.

Companion Virus

Companion viruses cannot live without joining an existing file or folder. To cause damage, the file or folder associated with the companion virus must be opened or run. The most popular examples of companion viruses are Terrax.1069, Stator, and Asimov.1539.

Network Virus

Network viruses are such types of viruses that spread usually through the Local Network Area (LAN) and the internet. The virus may further replicate through shared resources such as drives and folders.  The most dangerous network viruses are Nimda and SQLSlammer.


Multiple Characteristic Viruses

Apart from the above-mentioned viruses, there are some other viruses with different characteristics and different capabilities. They are not technically considered viruses, but they act like viruses and can cause the same damage. This includes malware, worm, spyware, adware, Trojan, and ransomware.

Summary

Depending on the type of computer virus, they can perform various types of actions. Some viruses are not so harmful, but some are. To protect your computer from viruses, install an anti-virus program and keep it up-to-date. There are several free anti-virus, but, it is recommended to always use good quality premium anti-virus software on your computer.



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