A virus is a an actively infectious computer program that places copies of itself into other application and programs, but not into data files. The reason why it gets attached itself to an executable program only, is that , in order to perform its destructive action , it has to get executed. every time it is executed, it replicates itself like cell division.

The reason why viruses cause so much fear is that they can destroy valuable data and programs, which could mean that you loose month of work. Typically on running the program to which a virus is attached, the virus infect all portion of memory and then infects the files on your hard disk & floppy or CD. so the next person using the same floppy disk or CD on a different computer spreads the virus to the new system.

In a network system, the damage can be worse, because the virus program spreads from one PC to another, thus infecting all the work station computers.

A virus remains dormant until the application or program which it is attached is executed on executing the program, the virus infects all the portion of memory and infects the files on your disk. when infectious program get copied from this system through floppy or CD, and is being used in another system, the virus is carried on and thus keep on spreading.