Computer virus is just a program, like any other programs. The only difference is just how it's designed, and the objective behind its design. Computer virus is not a biological virus, or anything close to it. So, computer virus, stored in a CD/DVD, will never infect other files/programs stored in other CD/DVDs, just because they are put close to each other in a shelf. It's not a biological virus after all.

The key differences that separates a computer virus to any other programs are
1. Computer virus can replicate itself
Computer virus is active when a program that contains the virus becomes active or launched. Once it's active, it stays in memory (until the computer is shut down). When in memory, it can spread itself to other programs, any media, including flash disk.

2. Computer virus annoys the users
This is the main objective why a computer virus is designed. To annoy others. The annoyance can be as simple as showing messages in dialog boxes on screen, to removing data files to frustrate the users.

There are some other variants to computer virus, with slight differences.
1. Worms
A worm replicates itself, but don't show any messages nor do anything destructive. It simply replicates itself to fill up the memory, slowing down computers. It infects other programs just like virus.
2. Trojans
Trojan horse is just a program, with a physical file, seems useful, but destructive. Since it's a program file, it doesn't infect like a virus, yet it annoys the users.

Creating an equation: Virus = Worm + Trojan

3. Spyware and Adware
The word 'ware' indicates a program, for the purpose shown at the heading of the name: 'spy' and 'ad'. 'Spy': it spies the computer, anything you're doing, and send the information somewhere else. 'Ad': it shows ads, it infects to spread the ads.

Knowing more about virus and its variants is important for computer users to prevent nasty things that might happen due to ignorance.

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