Most of us swap disks with friends and browse the Net looking for downloads. Rarely do we ever consider that we are also exchanging files with anyone and everyone who has ever handled them in the past. If that sounds like a warning about social diseases, its right along the same lines. Computer viruses are every bit as harmful and destructive, and come in a vast variety of types and strains. Computer viruses, can tear up your hard drive and bring down your system. However, unlike social diseases, computer viruses are almost always curable, and the cures for new strains are usually a matter of days away, rather than months or years.

A computer virus is a program designed to replicate and spread, generally with the victim being oblivious to its existence. Computer viruses spread by attaching themselves to other programs such as word processors or spreadsheets, or to the boot sector of the disk. When an infected file is activated, or when the computer is started from an infected disk, the virus itself is also activated. Many times a virus will lurk in a computers memory, waiting to infect the next program or disk that is activated.
What makes viruses so dangerous is their ability to perform an event. While some events are harmless like displaying a message on a certain date, and others annoying, like slowing performance or altering the screen display, some viruses can be disastrous by damaging files, and destroying data.

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Most viruses are created out of curiosity. There are those out there who create them out of malice, but far more of the creators are just meeting a challenge, to see if they can do it or not. A common type of virus would be a Trojan Horse, a destructive program disguised as a game, a utility, or an application. When run a Trojan Horse does something harmful to the computer system, while appearing to do something useful. A worm is also a popular type of virus. A worm is a program that spreads itself across computers, usually by spawning copies of itself in each computers memory. A worm might duplicate itself in one computers memory so often that it causes the computer to crash. A worm is introduced secretly into a host system either for fun, or with the intent to damage or destroy information. Now I will explain some of the more major viruses, and how they function.
Most of the thousands of viruses known to exist are file viruses. They infect files by attaching themselves to a file that is executable, usually the .EXE and .COM files that control applications and programs. The virus can insert its own code into any part of the file, misdirecting proper program execution so that it executes the virus code first, rather than the legitimate programs code. When the file is executed, the virus is executed first, infecting the system.

Most file viruses store themselves in memory. There they can infect other programs as they are executed. A simple file virus will overwrite and destroy a host file, immediately alerting the user to a problem because the software they are trying to run will not work. Because these viruses are immediately discovered they have less of an opportunity to spread. More dangerous file viruses can cause more subtle or delayed damage, and spread considerably before being detected.
There are only about 300 different boot sector viruses, but they make up for about 75 percent of all virus infections. Boot sector viruses include Stoned, the most common virus of all time, and Michelangelo, possibly the most notorious. These viruses are so prevalent because they are harder to detect, as they do not change a files size or slow performance, and are fairly invisible until their trigger event occurs, such as the reformatting of a hard disk. They also spread rapidly. The boot sector virus infects floppy disks and hard disks by inserting itself into the boot sector of the disk, which contain codes that are executed during the system boot process. Booting from an infected floppy disk allows the virus to jump to the computers hard disk. The virus executes first and gains control of

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