What is a Computer Virus ?
A potentially damaging computer programme capable
of reproducing itself causing great harm to files
or other programs without permission or knowledge
of the user.
Types of viruses :-
The different types of viruses are as follows-
1) Boot Sector Virus :- Boot sector viruses infect
either the master boot record of the hard disk or
the floppy drive. The boot record program responsible
for the booting of operating system is replaced by the
virus. The virus either copies the master boot program
to another part of the hard disk or overwrites it.
They infect a computer when it boots up or when it
accesses the infected floppy disk in the floppy drive.
i.e. Once a system is infected with a boot-sector virus,
any non-write-protected disk accessed by this system
will become infected.
Examples of boot- sector viruses are Michelangelo and Stoned.
2) File or Program Viruses :- Some files/programs,
when executed, load the virus in the memory and perform
predefined functions to infect the system. They infect
program files with extensions like .EXE, .COM, .BIN,
.DRV and .SYS .Some common file viruses are Sunday, Cascade.
3) Multipartite Viruses :- A multipartite virus is a computer
virus that infects multiple different target platforms, and
remains recursively infective in each target. It attempts to
attack both the boot sector and the executable, or programs,
files at the same time.When the virus attaches to the boot
sector, it will in turn affect the system’s files, and
when the virus attaches to the files, it will in turn
infect the boot sector.This type of virus can re-infect a
system over and over again if all parts of the virus are
not eradicated.
Ghostball was the first multipartite virus, discovered by
Fridrik Skulason in October 1989.Other examples are
Invader, Flip, etc.
4) Stealth Viruses :- These viruses are stealthy in nature
means it uses various methods for hiding themselves to
avoid detection. They sometimes remove themselves from the
memory temporarily to avoid detection by antivirus. They
are somewhat difficult to detect.When an antivirus program
tries to detect the virus, the stealth virus feeds the
anti virus program a clean image of the file or boot sector.
5) Polymorphic Viruses :- Polymorphic viruses have the
ability to mutate implying that they change the viral code
known as the signature each time they spread or infect. Thus an
antivirus program which is scanning for specific virus codes
unable to detect it's presense.
6) Macro Viruses :- A macro virus is a computer virus
that "infects" a Microsoft Word or similar application and
causes a sequence of actions to be performed automatically when
the application is started or something else triggers it.
Macro viruses tend to be surprising but relatively harmless.
A macro virus is often spread as an e-mail virus. Well-known
examples are Concept Virus and Melissa Worm.
