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•    It can infect files that are attached to e-mail messages or can be copied to disks and pen
                 drives. In this manner, it can spread from one computer to another.

            •    It can slow down a computer seriously and even stop it completely.


            WHAT A VIRUS CAN'T DO?
            •    It can't infect files which are written on a protected disk like a CD-ROM.

            •    It can't infect computer hardware. It can only infect computer software.

            SYMPTOMS OF A COMPUTER VIRUS

            A virus can affect a computer in different ways. However, the following problems are commonly
            noticed when a computer is infected with a virus.

            •    The computer runs slower than usual and takes longer time to load programs.

            •    The programs behave erratically and the computer stops responding or hangs frequently,
            •    The computer restarts every few minutes.

            •    New files keep appearing on the system. Files having strange names appear on the system.

            •    You are unable to access files or the contents of files get deleted.

            •    You notice unusual and strange error messages such as 'Your PC is stoned'.
            •    New icons appear on the desktop. Strange graphics are displayed on the monitor.

            •    The computer may crash.

            TYPES OF COMPUTER VIRUSES

            Computer viruses can be categorised in different ways. One of them is by their infection targets. If
            we group viruses by their target, we can divide viruses into the following categories:


            Program File Viruses

            These viruses infect executable files and programs, such as those with extensions, like .com, .exe,
            .ovl,  .dev,  .sys,  .scr,  .dll,  etc.  These  programs  get  loaded  in  the  computer  memory  during
            execution. The virus becomes active in the computer memory by replicating itself and infecting
            files on the disk. The examples of program file viruses are Sunday, Cascade, etc.


            Boot Viruses
            These viruses infect the boot records or the master boot record. They replace the boot record that
            is responsible for loading the operating system in the computer memory by copying it elsewhere.
            Boot viruses become active at the time of booting the machine. Examples of Boot virus are disk
            killer, stoned virus, etc.


                 Extra Bytes

                 The first PC virus was a boot sector virus named as Brain, released in 1986 by the Farooq Alvi
                 Brothers in Lahore, Pakistan.


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