A computer virus is a program designed to copy itself. The first computer virus was discovered in 1986. Today there are more than 54,000 known viruses and at least 200 are considered threats at any time. Combination threats are becoming more and more common also. A combination threat combines Internet worms, viruses, and Trojans. .
Worms differ from viruses because worms do not infect other files as viruses do. Instead of infecting other files a worm makes copies of itself over and over again. Worms can reduce system resources such as hard disk space or reduce bandwidth by spreading over network shares. Today's worms are also called internet worms or email worms. These worms send copies of themselves to recipients in infected users" address books. These types of worms can cause loss of productivity and revenue, as email servers can not handle all of the traffic. A Trojan is program that seems to be valid but in fact it does something to harm your computer. (www.antivirus.miningco.com).
Early viruses were pieces of code attached to common program such a game or word processor. A user might download an infected game from a bulletin board and run it. A virus like this is a small piece of code in a larger program. The virus is designed to run when the program gets executed. The virus loads itself into the memory, and looks around to see if it can find any other programs on the disk. If it finds one, it adds the virus's code to the program. Then the virus launches the real program. The user really has no way to tell that the virus has been run. However, the virus has already reproduced and infected other programs. The next time that any of these infected programs are ran the virus will infect other programs and the cycle will continue. If one of the infected programs is given to another person on a floppy disk or uploaded to a bulletin board then other programs get infected and this how the virus spreads.