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                      Computer virus

1.    what is a computer virus
2.    types of virus
3.    how can virus affect us
4.    how to detect virus
5.    how to clean virus
1  what is a computer virus
Since the first PC virus was found in 1986,the total number of virus has been rocketing to an enormous figure. As many may have known, computer virus is a piece of malicious program, which is able to affect the normal operation of a computer system.
Computer virus is A program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Viruses can also replicate themselves. All computer viruses are manmade. A simple virus that can make a copy of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems.
1983
Fred Cohen, while working on his dissertation, formally defines a computer virus as “a computer program that can affect other computer programs by modifying them in such a way as to include a (possibly evolved) copy of itself.”
In 1986 the frist real computer virus Two programmers named Basit and Amjad replace the executable code in the boot sector of a floppy disk with their own code designed to infect each 360kb floppy accessed on any drive. Infected floppies had “ Brain” for a volume label.

types of virus
1。boot sector virus
In the mid-1990s, boot sector viruses were the most prevalent virus types, spreading primarily in the 16-bit DOS world via floppy disk
Boot sector viruses work like this: by hiding on the disk, the virus is loaded into the memory  before system files are loaded. this allows it to gain complete control of DOS interrupts so that it can spread and cause damage.
For example:
Michelangelo is an infamous boot sector virus. It infects both boot sectors of diskettes and MBR(master boot record) of hard disks
2
1 file infector virus
File infector, also known as parasitic viruses, operate in memory and usually infect executable files. They copy themselves into other executable files and can remain in memory long after the virus has activated. A simple method of removing file infector virus is by using anti-virus software to scan and clean it. However , you cannot clean system files under windows because they are locked in windows mode. In this case, you have to boot your computer to DOS mode and then run the anti-virus software to clean the infected files.
For example:
CIH infects EXE files in windows 95/98 and has a couple destructive payloads that are triggered on the 26th day.
3
Multi-partite virus
Multi-partite viruses have characteristics of  both boot sector viruses  and file infector viruses
4
macro virus
macro virus infect macro utilities that accompany such applications as Microsoft  word and Excel. macro viruses are written in “every man’s programming language” such as visual Basic, and thes types of programming languages are relatively easy to create.
For example:
Julykiller spreads through MS word97 documents via VB macro.
5.
Trojan/Trojan horse
A Trojan or Trojan horse is a program that appears legitimate but performs some malicious and illicit activity when it is run. It may be used  as a hacker tool to leak a user’s password information or it may simply does not replicate itself Trojans often sneak in by attaching to a free game or other utility
For example
Back Orifice is a Trojan released in 1998.
6
Worm
Worms - A worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well.
For example
Discovered in June 1999, worm. ExploreZip is an auto-spamming worm. when executed, it sends email with itself as an attachment, to recipients in the windows address book. once the attachment is clicked upon in any windows  environment, this worm executes automatically.
7 E-mail viruses
An e-mail virus moves around in e-mail messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail address book.
3
How can virus affect us
Computer virus affects the health of your computer just like their biological counterparts make you sick. Typical payload of computer virus includes creating some annoyances, removing files from your hard disk and formatting your hard disk. It’s only the discovery of CIH virus that corruption to BIOS data has been added to the list of payloads
  Computer virus may seem remote from you. It may be true in the old days, when few of us have PC at home and virus spread slowly with the exchange of floppy disk. They may arrive from the shared files in the server, mails from your colleagues, files downloaded from the internet and
BBS. And worse still, some vendors deliver the machines/CD-ROMs with virus pre-installed. So, you are at stake.
For example
Melissa virus proved that virus could spread around the globe just within hours.
4
How to detect virus
New viruses are being developed every day. We can minimize the damage by identifying virus infections before they carry out their payload. the following lists some ways to detect virus infections:
(1)    watch out for any changes in machine behavior. any of the following signs could be symptoms of virus activity. programs take longer time than usual to execute or there is a sudden reduction in the available system memory or disk space
(2)    a memory-resident anti-virus software can be employed to continuously monitor the computer for viruses
(3)    install  and specified kill tools
(4)    scan your hard disk with anti-virus software. You should make sure that an up-to-date virus signature has been applied and you should update the signature at least once a week.
(5)    employ server-based anti-virus software to protect your network. You should also consider employing application-based anti-virus software to further protect your machine.
5
How to clean virus
Virus has been found? Don’t panic! Some pieces of advice about removing compute virus are given below:
(1)    all activities on the infected machine should be stopped(and it should be detached from the network)as the payload may be triggered at any time. Continuing the use of the infected machine helps the suspected virus spread further
(2)    recover from backup is the most secure and effective way to recover the files.
(3)    In some cases, you may recover the  boot sector, partition table and even the BIOS data using the emergency recovery disk.
(4)    In case you do not have the latest backup of your files, you may try to remove th virus using anti-virus software.