What is a internet virus and what does it do | Types of PC virus 

What is a internet virus and what does it do: Computer viruses are “deviant” programs that can cause destruction to computers that contract them. They are passed in two ways. The first way is via an infected diskette, such as one you get from a friend or a repair person, although it might also be a sales demo disk or even (in 3% of cases) a shrink-wrapped commercial disk.

The second way is via a network, such as files downloaded from an electronic bulletin board or as e-mail.
Different types of pc viruses

The virus then usually attaches itself to your hard disk. It might then display annoying messages (“your PC is stoned-legalize marijunana”) or cause Ping-Pong balls to bounce around your screen and knock away text. 

More seriously, it might add garbage to or erase your files or destroy your systems software. It may evade your detection and spread its havoc elsewhere. Viruses may take several forms. The two principal ones are the boot-sector virus and file viruses and others.

Boot sector virus: The boot sector is that part of the system software containing most of the instructions for booting or powering the system. The boot sector virus replaces these boot instructions with some of its own. When the system is turned on, the virus is loaded into main memory before the system. From there it is in a position to infect other files. Any diskette that is used in the drive of the computer then becomes infected. When that disk is moved to another computer, the contagion continues.
File virus: File viruses attach themselves to executable files-those that actually begin a program. (These file have the extensions .com and .exe) when the program is run, it starts working, trying to get into main memory and infecting other files.
Worm: The worm spreads by replicating itself. It travels through networks to different computer systems, and then makes many copies of itself, wasting vast amounts of computer time.
Logic bomb: Logic bombs, or simply bombs, differ from other viruses in that they are set to go off two months after he left. Designed to erase an inventory tracking system, it was discovered only by chance.
Trojan horse: The Trojan horse covertly places destructive instructions in the middle of a legitimate program, such as a computer game. When you run the program, the Trojan horse goes to work, ding damage while you are blissfully unaware.
Polymorphic virus: A polymorphic virus, of which there are several kinds can mutate and change form just as human viruses can. These are especially troublesome because they can change their profile, making existing antiviral technology ineffective.
Virus mutation engines: A virus mutation engine allows programmers to actually create polymorphic viruses. One, called Virus Creation Laboratory, has a user interface with pull-down menus, making it easy for even novices to create their own viruses.
There have been many strains of viruses in recent years, some of them quite well known (stoned, Jerusalem, Lehigh, Pakistani brain). Most famous was Michelangelo, so named because it was programmed to erase hard disks on the artist’s birthday, March 6, 1992. Michelangelo was supposed to cause a million computers to crash, but on March 7 it was found that only a few thousand were damaged. Does this mean that the threat was oversold mainly by makers of antivirus software? Antivirus software scans a computer’s hard disk and main memory to detect viruses and sometimes to destroy them. Since the Michelangelo scare, the appearance of polymorphic viruses, virus mutation engines, and other new forms shows that this threat must still be taken seriously.

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