What Is A Computer Virus ? Causes of Computer Virus ? Types of Computer Virus

What Is A Computer Virus?

A Computer Virus is a malicious software program “Malware” that can infect a computer by modifying or deleting data files, boot sector of a hard disk drive or causes a software program to work in an unexpected manner”.

A computer virus resides on a host computer and can replicate itself when executed. Virus can steal user data, delete or modify files & documents, records keystrokes & web sessions of a user. It can also steal or damage hard disk space, it can slowdown CPU processing.

Activation of Virus

When the computer virus starts working, it is called the activation of virus. A virus normally runs all the time in the computer. Different viruses are activated in different ways. Many viruses are activated on a certain data. For example, a popular virus “Friday, the 13th is activated only if the date is 13 and the day is Friday.

According to Wikipedia “Computer viruses cause billions of dollars’ worth of economic damage each year, due to causing systems failure, wasting computer resources, corrupting data, increasing maintenance costs, etc”.

Causes of Computer Virus

The following are the main causes of a Computer Virus.

Infected Flash Drives or Disks

Flash drives and disks are the main cause of spreading viruses. Flash drives and disks are used to transfer data from one computer to other. A virus can also be copied from one computer to other when the user copies infected files using flash drives and disks.

Email Attachments

Most of the viruses spread through emails. Email attachment is a file that is sent along with an email. An email may contain an infected file attachment. Virus can spread if the users opens and downloads an email attachment. It may harm the computer when it is activated. It may destroy files on the hard disk or may send the virus automatically to all email addresses saved in the address book.

Infected / Pornography websites

Thousands of insecure websites can infect computer with viruses. Most of the websites with pornographic materials are infected, so by visiting these websites the user’s computer also gets infected by virus. These websites are developed to spread viruses or other unethical material. The virus is transferred to the user’s computer when this material is downloaded. These websites may access the computer automatically when the users visit them.


Virus can spread if an infected computer is connected to a network. The internet is an example of such network. When a user downloads a file infected with virus from the internet, the virus is copied to the computer. It may infect the files stored on the computer.

Pirated Software

An illegal copy of software is called pirated software. Virus can spread if user installs pirated software that contains a virus. A variety of pirated software is available in CDs and from the internet. Some companies intentionally add virus in the software. The virus is automatically activated if the user uses the software without purchasing license.

Types of Computer Virus

The following are some well-known viruses.


It is a worm that infects a computer running Microsoft IIS server. This virus launched DOS attack on White House’s website. It allows the hacker to access the infected computer remotely.


It is a worm that spreads itself using different methods. IT damages computer in different ways. It modified files, alters security settings and degrades performance.


It is distributed as an email attachment. It may delete files, degrade performance and send the files to anyone.


It is a virus that is distributed as an email attachment. IT disables different safeguards in MS Word. It sends itself to 50 people if Microsoft Outlook is installed..


It corrupts data from the hard disk.


It is transferred from one MS Word file to other if both files are in memory.


It is also transferred as an email attachment. It saves the file in template directory instead of its original location.


It encrypts hard disk so only the virus may read the data. It displays One_Half on the screen when the encryption is half completed.

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