One may think of several explanations of what is a computer virus. The simplest of them is a commonplace one fit for a housewife in what has never seen a computer in her life, but knows that It exists, and that It is prone to virus infection. Such an explanation can be given rather easily, unlike the other one, meant for an expert programmer. So far I don't think I can give an exact definition of a computer virus and set a clear margin between programs basing on a principle "virus - non-virus".
Explanation for a Housewife
This explanation will be given on the example of a desk clerk working exclusively with papers. The idea of an explanation like this belongs to D.N.Lozinsky.
Let's imagine a desk clerk coming to work every day to his office. Everyday he finds a stack of papers with a list of tasks which he must fulfill during his working day. He takes the top paper from the stack, reads the instructions of the superior, follows them carefully, then throws "used" papers into waste basket. Suppose a bad guy sneaks into the office and inserts a paper into the stack with his own task which goes like this:
· "Copy this paper two times and put the copies into neighbors' stacks". What will the desk clerk do? He will copy this paper twice, destroy the original one and continue to the next paper in the stack, i.e. will go on working as usual. What will his neighbors do, being as careful workers as he is, when they find a new task? They will do the same thing as the first one did: copy the paper twice and give it to other desk clerks. Altogether we have four copies of the paper already, and the paper will continue to be copied and transferred to other people.
This is approximately this scenario according to which the computer virus works, with programs instead of papers stacks and computers instead of desk clerks. A computer, like a desk clerk, carefully fulfills all the commands contained in a program (task lists), starting from the first one. If the first one like "copy my body into two other programs", the computer will do so, and the virus command will now be in two other programs. When the computer starts running other "infected" programs, the virus will continue to spread all over the computer in a similar manner.
In the above example about a desk clerk and his office our paper virus does not check whether another stack of papers is infected or not.