Page 39 - Viva ICSE Computer Studies 7 : E-book

attach itself to other applications or fi les, and infect a computer. Most of the viruses stay active
in the computer’s memory until you turn off the computer.

Types of Computer Viruses

There are many types of viruses. Let’s learn about them.

Boot Sector Viruses

A hard disk or hard drive consists of smaller sections known as sectors. The fi rst sector is known
as boot sector. The boot sector is the area of a hard disk that stores valuable information. Boot
sector virus damages this boot sector of the hard disk or fl oppy disk. It gets activated at the
time of booting. Stone virus is an example.

Trojan Horses

Trojan horse is a coded program that masks the existence of a virus or malware by making it
appear friendly. It steals information from the computer. Trojan horse can appear as a game
and mobile application. Backdoor and Exploit are some examples.

Program File Virus

A program fi le virus infects the program or executable fi les like the ones with extension .com,
.exe and .sys. It replicates itself and infects fi les. It either causes permanent damage or makes
the fi les unreadable. Sunday is an example.

Worm

A worm is a malicious program that duplicates itself. It creates copies of itself and fi lls up the
disk space with these copies. It slows down the system. Michelangelo and MSBlast are some
examples.

Polymorphic Virus

A polymorphic virus appears in diff erent forms. It infects a fi le and changes its binary pattern to
avoid detection by an antivirus program. Elkern and Marburg are some examples.

Network Virus

A network virus enters the system through a network. It generally
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multiplies through shared resources and spreads through local
area network and sometimes through the Internet. It infects one MyDoom is considered
computer in a network, then moves on to the next computer as the most expensive
and the cycle keeps on going. SQL Scanner is an example. virus in the world.

Directory Virus

A directory virus is designed to change the location of the infected fi le. So when you execute
it, you will never know that you have run a virus. Dir-2 is an example.

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