In the movie "Independence Day," it takes everyone a while to figure out how to fight back against the aliens. Not only do the aliens outnumber the earthlings, they're also equipped with far superior technology. So, it's pretty amazing when the "good guys" finally figure out how to take out the "bad guys." And, the plan seems remarkably simple: Give the aliens a virus -- a computer virus -- that will cripple their technology! That exact scenario is actually pretty far-fetched. However, the concept itself is not.

Computer viruses are actually a very real and very serious threat to our own technology. A properly engineered virus can have a staggering effect. For example, experts estimate that the Mydoom worm infected approximately a quarter-million computers in a single day in January 2004.

Computer viruses are called viruses because they share some of the traits of biological viruses. A computer virus passes from computer to computer. A biological virus passes from person to person. A biological virus is not a living thing. It is a fragment of DNA inside a protective jacket. Unlike a cell, a virus has no way to do anything or to reproduce by itself -- it is not alive. Instead, a biological virus must inject its DNA into a cell. The viral DNA then uses the cell's existing machinery to reproduce itself. A computer virus must piggyback on top of some other program or document in order to be executed. Once it is running, it is then able to infect other programs or documents. Obviously, the analogy between computer and biological viruses stretches things a bit, but there are enough similarities that the name sticks.

Early viruses were pieces of code attached to a common program like a popular game. A person might download an infected game and run it. A virus like this is a small piece of code embedded in a larger, legitimate program. This type of virus is designed to run when the legitimate program is executed. So in this case, that would be the first time the game is played. The virus loads itself into memory and looks around to see if it can find any other programs to infect. If the virus can find one, the virus infects the program by adding its own code to the unsuspecting program. Then the virus launches the "real program." The user really has no way to know that the virus ever ran. Unfortunately, the virus has now reproduced itself, so two programs are infected. The next time either of those programs executes, they infect other programs, and the cycle continues. Now, let's say one of the infected programs is shared with another person's computer. This is how other programs get infected. This is how the virus spreads.

Besides spreading, most viruses also have some sort of destructive attack phase where they do some form of damage. Some sort of trigger will activate the attack phase, and the virus will then "do something" -- anything from printing a silly message on the screen to erasing all of your data. The trigger might be a specific date, or the number of times the virus has been replicated, or something similar.

Worms A worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well.