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 VirusesThis 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 VirusThis 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 VirusMacro 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.
EncryptedEncryption 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 VirusesCompanion 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.
WormsA 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 HorsesAnother 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].
