What is Computer Virus? How do Viruses Spread?

A virus is a piece of malicious code that interferes with the normal functioning of a system and damages the way it operates. It is self-replicating in nature and copies itself or attaches itself to other legitimate programs and runs when that program runs.

Viruses are created with the intention of stealing sensitive user data, gaining admin control or infecting vulnerable systems. Hackers use them as weapons to fulfill their malicious purposes.


Viruses spread and infect a system without the user’s knowledge. It is easier to infect a system with viruses. It can intrude a system through infected email attachments, malicious websites or links, or shared media like USB or in any other way.

Viruses can severely impact your devices by disrupting the system software, modifying essential functions or encrypting data. Protective measures must be undertaken in advance to prevent one’s system from being attacked.

In this article, we will go through how virus originated, how they attack a system, how they spread, their types and examples as well as how to protect against them. Let’s begin.

What is the history of computer virus?

The first known computer virus was developed by Robert Thomas, an engineer at BBN technologies, in 1971. The virus, named “Creeper”, was an experimental program carried out by Thomas as infected mainframes on ARPANET.

“I’m the Creeper. Catch me if you can”  was the teletype message displayed on the screens.

However, the first wild computer virus to be tracked down was “Elk Cloner”, developed by a teenager named Richard Skrenta in 1982. It infected Apple II operating systems via floppy disks.

Though virus was designed as a prank, it showed the way how malicious programs could be installed on computer’s memory and prevent users from removing them.

Fred Cohem coined the term “computer virus” in 1983 and popularized it through his paper named “Computer Viruses – Theory and Experiments” detailing about the malicious programs in his work.

How does virus affect the computer?

A virus remains dormant after being successfully attached to any other program on the device until its code is executed. When the infected program runs, the virus code also runs, thus infecting the system.

So, even after entering a system, viruses can stay dormant without showing any danger sign or symptom.

If a virus affects your computer, it can also affect many other devices on the same network. It can cause severe damages by stealing user data, logging keystrokes, spamming contacts, corrupting files, etc.

The way the virus impacts your system depends on its nature. Some may be a simple prank whereas some may cause profound damage to the system.

Usually, viruses delete files and display messages. Some of them can be worse by inflicting permanent damage to the hard disk.

How do viruses spread?

There are many ways in which a virus can spread. Some common methods are discussed below.

Through email

Email is one of the ubiquitous ways through which virus spreads. Often, viruses are sent as email attachments and are installed on a system as soon as the user clicks them.

Also, viruses are being transported in the HTML body of the email itself. Rich graphics are also used to hide them and trick the users.

Instant messages

Several instant message services like Facebook Messenger, Skype, etc can be used for spreading viruses through chats.

By sending an infected link or malicious attachment through chats, people are also exposed to virus attacks.

Viruses through social media and IM services spread faster as they reach out to thousands of users in a very short time.

File sharing

There are peer-to-peer file sharing services like DropBox, ShareFile, etc which help in spreading viruses too. They sync folders of any computer linked to a specific account.

So, when a user uploads an infected file in the file-sharing account, anyone with access to that shared folder can be affected by the virus.

Another file sharing service called Google Drive scans uploaded files for viruses but upto file size of 25MB only. Therefore, if the virus is larger, then it is undetected.

Software download

Sometimes, viruses lie embedded in free software or pose as useful free software, thus tricking the users. Downloading such softwares can affect your system.

Hackers use pop-ups, adware, and social engineering techniques tempting you to install such free softwares. Once you give in, your system falls prey to their plans.

Backdated software

Softwares are periodically updated to improve any weakness or vulnerability in its earlier versions. Unpatched softwares, thus, are an excellent entrance for virus attacks on any system if someone leaves them non-updated.

What are the different types of viruses?

There are many types of viruses found in the cyber world. Let’s find out what are they.

Boot Sector Virus

The boot sector is part of the hard drive of your system which helps in loading the operating system, like Microsoft Windows.

A boot sector virus infects the master boot record and loads the virus in the computer memory during startup.

Though it’s rare now, it is very complex and difficult to remove if it attacks the system. It is transported through pluggable devices like USB keys, floppy disks, CD-ROMS, etc.

Some examples of boot sector viruses are “Elk Cloner”, “Stoned”, “Parity boot”, etc.

Direct Action Virus

It is the simplest kind of virus and the easiest one to remove as well. Direct action virus remains attached to specific files on the system, without affecting user experience. It usually affects COM or EXE files.

Once it’s done, it deletes itself. Direct Action virus is also called the non-resident virus. Examples include Win64.Rugrat, Vienna, etc.

Resident virus

These viruses remain on the computer, even if the original infector is removed. Some of them damage the computer pretty fast whereas some of them do it slowly. They are tough to detect.

Examples include Jerusalem Virus (hides in RAM and deletes programs on Friday 13th), Magistr virus (affects CMOS, BIOS and sends infected emails to your email list), etc.

Multipartite virus

This is a versatile virus affecting both the boot space and files on your computer together.

Even if you remove it from all the files, it still hides in the boot sector and can attack again. If you remove it from boot space, it can jump back through an infected file.

Examples include junkie virus (transported as fake Pacific Strike game), Tequila virus (avoided files with ‘v’ and ‘sc’ in their names), etc.

Polymorphic virus

This is very hard to detect as after infecting a system, the polymorphic virus changes its signature pattern while replicating.

Examples are Satanbug virus (has 9 encryption levels and hard to detect), VirLock virus (a kind of ransomware), etc.

Macro Virus

The macro virus targets the macro language commands in applications like Microsoft Word.

The macros are keystrokes, embedded in the documents or saved sequences for commands, in MS Word. The macro viruses are developed to add their malicious code to the genuine macro sequences in a Word file.

For example, Melissa was one such macro virus which spread through email attachments and made way to one’s Outlook mail client in your PC. It then mailed the user’s first 50 contacts as well as slowed down the servers.


What are some ways to prevent virus attacks?

There are several techniques to prevent virus attacks from affecting your systems.

  • Install well-known and reliable antivirus software for detecting and removing virus as well as malwares from your system. Avast Free antivirus, Windows BitDefender, McAfee, Avira, Kaspersky Labs, etc are famous antivirus softwares.
  • Avoid clicking on any advertisements or pop-ups while online. Most of the time, viruses and malwares hide in them and get installed on your system with one click.
  • Do not open or click email attachments from unknown senders. Viruses often spread as infected attachments.
  • Do not click on any suspicious link available through instant message services or social media platforms. Viruses spread in a superfast way through these mediums and hence are successful in attacking a large number of users too.
  • Try not to download any free software. Viruses can be embedded inside free softwares and invade your system when you download such software.
  • Always keep your software and applications updated. Unpatched softwares are great opportunities for viruses to enter a system because they contain weaknesses or vulnerabilities.


A virus is a kind of malicious code which attaches itself with other legitimate documents or programs on a system. Usually, they remain dormant and execute when the program, they are attached to, runs.

They are self-replicating in nature and damages the way a system operates. They can delete files, modify them, encrypt them as well as damage hardware of the system.

There are different types of viruses like the resident virus, multipartite virus, direct action virus, boot sector virus, etc. Viruses spread through infected email attachments, malicious links, unpatched softwares, etc.

To protect against virus attacks, it is better to adopt protective measures in advance. Avoid opening an email attachment from unknown senders, ignore clicking unknown links, update your softwares and install antivirus software to bolster the security of your system.


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