Computer Virus

Prepared by Miss Samreen KhizerFor Grade V

Computer Virus

computer virus is a program ( or software ) , which is written

intentionally to disrupt or destroy the information

contained on the disk Or

A computer virus is any destructive computer program

How does computer virus spread?

Viruses spread by the use of infected data or programs residing in the disk while copying them from one system to another.

They also spread by getting attached to e-mail messages or by flow of other forms of information on the internet.

When the computer is switched OFF , the virus gets removed from the primary memory but not from the disk or file that has been infected. When the computer is switched ON , the virus gets activated again and starts infecting other programs and files

Classification of computer viruses

Computer viruses are classified into two categories.

•Boot Sector Virus

•File Virus

•Program Virus

Boot Sector Viruses

Boot Sector Viruses

Boot viruses infect the boot area of floppies , hard disk and become resident and active at the time of booting.

Booting means starting the computer and loading the operating system

A boot sector virus replaces the information in the boot sector by some different code.A boot sector virus is executed every time a computer is switched ON and infects all the files on the disk.For Example :- Michelangelo , Junkee and Ohio etc

File Infecting Viruses File infecting virus attach themselves to executable program files with extension name such as .com , .exe , .sys , .sys etc.For Example :- Friday the 13th , Enigma , Nemesis etc

Program Viruses

Program viruses are different from boot viruses in that they usually attempt to destroy and infect data files instead of erasing the hard disk , filling the hard disk with “garbage”Files , or using all available memory .For Example : Macro virus , WORM , Trojan Horse

Damaging cause by virus and real world experience

•In 1999 , the Melissa virus resulted in many of the world’s largest •companies having to shut down their e-mail systems completely.

•In the year 2000, millions of people opened an attachment to an •e-mail headed ‘I Love U” and thus infected their computers •and spread this virus. The virus sent itself to everyone in each •computer’s e-mail address book and then started damaging •files on the recipient’s computer.

Effects Of Viruses.Different viruses can affect a computer system in different ways. Some of the harmful effects that a computer virus can cause are discussed below.•A computer virus can infect software and not hardware.

•It can infect executable files , i.e files with .exe extensions.

•It can infect the boot records , the files needed for starting the computer.

•It can infect and corrupt the data files .

•It can alter the contents in a file.•It can increase the size of a file.

•It can change all the files names to a single name.

•It can overwrite the entire directories.

•It can destroy parts of files ( programs or data ) by creating bad sectors.

•It can erase a specific program or data.

•It can format the disk resulting in loss of all data and programs.

•It can slow down the system performance.

•It can lead to periodic system hang-ups.

Precautions To Protect Your System

DO ….

Do ….•Write-protect the floppies when allowing data to be copied from them in other computer system.

•Scan all the floppies and USB Flash Drive’s before using them with antivirus software to detect for possible viruses present in them.

•Reformat the used floppies.

•Keep periodic backup of all data and programs files.

•Update your antivirus program periodically.

Restrict unauthorized use of the computer system

Don’t ……..

•Use floppies without scanning them for viruses.

•Ever run unknown programs without making sure they have no virus.

•Allow everyone to use the computer system.

Anti-Virus

Anti-virus software programs are developed to ensure a virus-free environment. They are designed to detect and remove viruses from the infected programs or files in the computer system.

Types of Antivirus Programs

Antivirus programs are of two types:

•Scanners

•Antidotes

Scanners

Scanners are the programs that scan the disks and detect many viruses. These programs check the disks for some known viruses. VirusScan by McAfee Associates is an example of a virus scanner. It scans the disks and informs the user if it finds any infected files. Then you can either delete the infected file or get it cleaned by an Antidote.

Antidotes

Antidotes are the program which clean the diskettes of the viruses they are programmed to detect . Cleaning involves deleting the virus code from the infecting file.

In case of boot-sector/partition table virus , it requires deleting the virus code and copying the boot programs back to their proper place.

Norton Anti-Virus and McAfee are examples of such software.

Anti-virus Software Most of the Anti-virus software available these days perform scanning as well as removal of viruses.Anti-virus always resides in the memory so that it detects the viruses at the time of their entry and prevents them from doing any damage to the files or system. Some popular antivirus softwares are :-•Norton Anti Virus•McAfee•Dr. Solomons•Smart Dog