Definition:
A 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 man-made. 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.
Since 1987, when a virus infected ARPANET, a large network used by the Defense Department and many universities, many antivirus programs have become available. These programs periodically check your computer system for the best-known types of viruses.
Some people distinguish between general viruses and worms. A worm is a special type of virus that can replicate itself and use memory, but cannot attach itself to other programs.
Advantages
-Takes advantage of the resources available on an infected system to do some or all of the following: to propagate itself via floppy drives, email or network connections; to destroy data; and possibly to embarrass the owner of the system by distributing documents found on the victim system's hard drive.
-Viruses can be written to take advantage of a particular program or operating system's features so as to make it more likely that they will be inadvertently run.
Disadvantages
-Viruses can cause many problems on your computer. Usually, they display pop-up ads on your desktop or steal your information. Some of the more nasty ones can even crash your computer or delete your files.
-Your computer gets slowed down
-Some the applications won't start, You can not see some of the settings in your OS
-Many "hackers" get jobs with software firms by finding and exploiting problems with software.