Know About Virus

A computer virus is a computer program that can copy itself and infect

a computer without permission or knowledge of the user

Here is the List of virus:

Resident Viruses
This type of virus hides permanently in the RAM memory. From here it

can control and intercept all of the operations carried out by the

system: corrupting files and programs that are opened, closed, copied,

renamed etc.
Examples include: Randex, CMJ, Meve, MrKlunky.

Overwrite Virus
This type of virus is characterized by the fact that it deletes the

information contained in the files that it infects, rendering them

partially or totally useless once they have been infected.

Infected files do not change size, unless the virus occupies more

space than the original file, because instead of hiding within a file,

the virus replaces the files content.
Some examples of overwrite viruses include: Way, Trj.Reboot,

Trivial.88.D.

Macro Virus
Macro viruses infect files that are created using certain applications

or programs that contain macros. These include Word documents (DOC

extensions), Excel spreadsheets (XLS extensions), PowerPoint

presentations (PPS extensions), Access databases (MDB extensions),

Corel Draw etc.

A macro is a small program that a user can associate to a file created

using certain applications. These mini-programs make it possible to

automate series of operations so that they are performed as a single

action, thereby saving the user from having to carry them out one by

one.
Some examples of macro viruses: Relax, Melissa.A, Bablas, O97M/Y2K.


Encrypted
Encryption is a technique used by viruses so that they cannot be

detected by antivirus programs.

The virus encodes or encrypts itself so as to be hidden from scans,

before performing its task it will decrypt itself. Once it has

unleashed its payload the virus will then go back into hiding.
Examples of encrypted viruses include: Elvira, Trile.

Companion Viruses
Companion viruses can be considered file infector viruses like

resident or direct action types. They are known as companion viruses

because once they get into the system they "accompany" the other files

that already exist. In other words, in order to carry out their

infection routines, companion viruses can wait in memory until a

program is run (resident viruses) or act immediately by making copies

of themselves (direct action viruses).

Some examples include: Stator, Asimov.1539, Terrax.1069.

Worms
A worm is a program very similar to a virus; it has the ability to

self-replicate, and can lead to negative effects on your system and

most importantly they are detected and eliminated by antiviruses.

However, worms are not strictly viruses, as they do not need to infect

other files in order to reproduce.

Worms can exist without damaging files, and can reproduce at rapid

speeds, saturating networks and causing them to collapse.

Some examples of worms include: PSWBugbear.B, Lovgate.F, Trile.C,

Sobig.D, Mapson.

Trojans or Trojan Horses
Another unsavory breed of malicious code are Trojans or Trojan horses,

which unlike viruses do not reproduce by infecting other files, nor do

they self-replicate like worms.

Trojans work in a similar way to their mythological namesake, the

famous wooden horse that hid Greek soldiers so that they could enter

the city of Troy undetected.

They appear to be harmless programs that enter a computer through any

channel. When that program is executed (they have names or

characteristics which trick the user into doing so), they install

other programs on the computer that can be harmful.
Some examples of Trojans are: IRC.Sx2, Trifor.(source:virus portal)

E-mail viruses -
An e-mail virus travels as an attachment to e-mail messages, and

usually replicates itself by automatically mailing itself to dozens of

people in the victim's e-mail address book. Some e-mail viruses don't

even require a double-click -- they launch when you view the infected

message in the preview pane of your e-mail software [source: Johnson].