What is a computer virus?

It is an executable code able to reproduce itself. Viruses are an area of pure programming, and, unlike other computer programs, carry intellectual functions on protection from being found and destroyed. They have to fight for survival in complex conditions of conflicting computer systems. That's why they evolve as if they were alive. They may have complex crypting/decrypting engines in order to carry out processes of duplicating, adaptation and disguise.

It is necessary to differentiate between reproducing programs and Trojan horses. Reproducing programs will not necessarily harm your system because they are aimed at producing as many copies (or somewhat-copies) of their own as possible by means of so-called agent programs or without their help. In the latter case they are referred to as "worms". Meanwhile Trojan horses are programs aimed at causing harm or damage to PCs.

As you see, there are different types of viruses, and they have already been separated into classes and categories. For instance: dangerous, harmless, and very dangerous. No destruction means a harmless one, tricks with system haltsmeans a dangerous one, and finally with a devastating destruction means a very dangerous virus.

But viruses are famous not only for their destructive actions, but also for their special effects, which are almost impossible to classify. Some virus-writers suggest the following: funny, very funny and sad or melancholy (keeps silence and infects). But one should remember that special effects must occur only after a certain number of c ontaminations. Users should also be given a chance to restrict execution of destructive actions, such as deleting files, formatting hard disks. Thereby virus can be considered to be a useful program, keeping a check on system changes and preventing any surprises such as of deletion of files or wiping out hard disks.