Computer Virus – A brief Introduction

computer virus

What is a Computer Virus?

Computer virus is a man made destructive computer program or code that is generally loaded onto a computer system without the knowledge of a user and causes unauthorized and unwanted changes to the information stored on the computer. The term computer virus may be applied to software, code, code blocks, and code segments which perform illegal damaging functions in the computer system.

Computer virus assaults computer systems through distribution via e-mail attachments, software downloads, and even some kinds of advanced web scripting. It is important to note that all software that causes damage is not virus. Popular legal software may cause damages because of certain unfixed bugs. But viruses are made for infecting other programs and troubling others, which these software are not.

Computer viruses are called viruses because they share some of the traits of biological viruses. A computer virus transmits from computer to computer like a biological virus transmits from person to person.

What does a Computer Virus can do?

Computer viruses may cause various kinds of damages to a computer system. The damage caused by a virus depends on the type of virus. Following are some key damages a computer virus can cause:

  • Computer virus can damage the computer system by corrupting programs, erasing files, scrambling data on the hard disk, attacking FAT, attacking partition table, and even by formatting the hard disk.
  • Some computer viruses consume excessive amount of computer resources thereby preventing users from performing the desired tasks or leading to hang the computer.
  • Some viruses destroy all contents from the system hard disk which causes the computer system to completely inoperable.
  • Some viruses interfere with display or audio system by scrambling characters and displaying misleading text, video or audio messages to users.
  • The viruses may attack client computers, network servers, network traffic, or even the whole network. If an infected computer system is part of a network then viruses can be transmitted to other systems in the network causing widespread damage.
  • Some viruses may use excessive amount of network resources which can choke the network traffic.
  • Some viruses can steal data from your computer and send it to other computer over the internet.

Types of Computer Viruses:

There are various types of viruses causing various types of disturbances in a computer system. Some viruses alter or destroy data on the disk, some viruses scramble characters on video display screen, some others display misleading messages to users whereas some others consume computer/ network resources thereby making the computer/ network slow.

Following are some well known and popular types computer viruses:

Boot-Sector Viruses:
Boot sector virus places its own codes and commands into a computer’s DOS boot sector or Master Boot Record (MBR). When this type of virus has infected a system, the MBR is usually corrupted and a computer’s boot sequence is changed. Boot sector viruses can be dangerous and prolific because they are loaded onto a computer every time one starts up, and in time, they can spread to other readable disks. Boot sector viruses can cause booting problems, start up problems, problems with retrieving data, computer performance instability, inability to locate hard drive, etc.

File Viruses:
File viruses infect executable files by inserting their code into some part of the original file so that the malicious code can be executed when the file is accessed. An overwriting file virus is one that overwrites the original file entirely, replacing it with the malicious code.

Email Viruses:
Email virus is a type of computer script or code sent as an email attachment, which if activated can cause an unexpected harmful effect in the computer system like destroying the system and program files in the hard disk.

Network Viruses:
Network virus uses network protocols to spread and infect other computers on a computer network. Network viruses have the ability to quickly degrade the performance of a network, totally disabling critical devices, programs and network connections.

Macro Viruses:
Macro viruses are encoded as a macro embedded in a document. Many applications, such as Microsoft Word and Excel, support powerful macro languages. These applications allow you to embed a macro in a document, and have the macro execute each time the document is opened.

Multipartite Viruses:
Multipartite virus is a computer virus that infects and spreads in multiple ways. It attempts to attack both the boot sector and the executable files or programs at the same time. When multipartite virus attacks to the boot sector, it will in turn affect the systems files, and when it attacks to the system files, it will in turn infect the boot sector.

Memory Resident Viruses:
A memory resident virus stays in memory after it executes and even after its host program is terminated. It infects other files when certain conditions are met.

Malware, is short for malicious software, is software used or programmed by attackers to disrupt computer operation, gather sensitive information, or gain access to private computer systems. It can appear in the form of code, scripts, active content, and other software. Malware includes computer viruses, ransomware, worms, trojan horses, rootkits, keyloggers, dialers, spyware, adware, malicious BHOs, rogue security software and other malicious programs; the majority of active malware threats are usually worms or trojans rather than viruses.

Trojan Horse Viruses:
Trojan Horse Viruses are a program or utility that appears to be something useful or safe. But in reality it will performing background tasks such as giving access to your computer or sending personal information to other computers. Trojan horses are one of the most common methods a criminal uses to infect your computer and collect personal information from your computer.

Worms are malicious programs that make copies of themselves again and again on the local drive, network shares, etc. The only purpose of the worm is to reproduce itself again and again. It doesn’t harm any data/file on the computer. Unlike a virus, it does not need to attach itself to an existing program. Worms spread by exploiting vulnerabilities in operating systems. Due to its replication nature it takes a lot of space in the hard drive and consumes more CPU uses which in turn makes the PC too slow also consumes more network bandwidth.

Adware is a software application in which advertising banners are displayed while any program is running. Adware can automatically get downloaded to your system while browsing any website and can be viewed through pop-up windows or through a bar that appears on a computer screen automatically. Adwares are used by companies for marketing purpose.

Spyware is a type of program that is installed with or without your permission on your personal computers to collect information about users, their computer or browsing habits tracks each and everything that you do without your knowledge and send it to remote user. It also can download other malicious programs from internet and install it on the computer. Spyware works like adware but is usually a separate program that is installed unknowingly when you install another freeware type program or application.

Polymorphic Viruses:
Polymorphic virus is a computer virus that creates copies of itself, with variations in each copy to fool a virus detection program and user. The variations are typically different encryption methods in the virus file copies, which makes it more difficult for a virus detection program to detect and remove a polymorphic virus from a computer.

Metamorphic Viruses:
Metamorphic viruses rewrite themselves completely each time they infect, and so that each succeeding version of the code is different from the preceding one. The code changes makes it difficult for antivirus software programs to recognize that different iterations are the same malicious program.

Stealth viruses:
These types of viruses use different kind of techniques to avoid detection. They either redirect the disk head to read another sector instead of the one in which they reside or they may alter the reading of the infected file’s size shown in the directory listing.

A rootkit is a stealthy type of software designed to hide the existence of certain processes or programs from normal methods of detection and enable continued privileged access to a computer. Rootkits can intercept data from terminals, network connections, and the keyboard.

Spamming is a method of flooding the Internet with copies of the same message. Most spams are commercial advertisements which are sent as an unwanted email to users. Spams are also known as Electronic junk mails or junk newsgroup postings. These spam mails are very annoying as it keeps coming every day and keeps your mailbox full.

Tracking cookies:
A cookie is a plain text file that is stored on your computer in a cookies folder and it stores data about your browsing session. Cookies are used by many websites to track visitor information A tracking cookie is a cookie which keeps tracks of all your browsing information and this is used by hackers and companies to know all your personal details like bank account details, your credit card information etc. which is dangerous.

How to protect a Computer System from Viruses?

When you’re online or connected to network, you expose your computer for virus attack. But it doesn’t mean that you can’t connect to internet or other networks to avoid virus attack. There are a lot of antivirus softwares available to protect a PC from getting a virus, or clean a virus. Also to prevent any virus attack, don’t download anything from anyone you don’t know.

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