Viruses are simply programs written to create copies of themselves and to attach these copies to other programs. The only way any virus can infect your PC is when one of these programs executed or by booting your PC from a diskette containing an infected boot sector. Under some circumstances it can actually explode and destroy organisms.
Computer viruses are computer programs, just like any other programs.
If such a program is transferred to your PC, other programs on your PC will become infected since it will spread from program to program or from disk to disk. A computer virus may be enter your computer through your actions, or those of others. For example, any time you download a program from another computer, or run a program from a diskette you've obtained from someone else. Viruses spread rapidly through networked computers, able to infect many computers rapidly. Viruses come in a great many different forms, but they all potentially have two phases to their execution, the infection phase and the attack phase.