A computer virus is a program that is intentionally designed to modify, disrupt, or damage files and programs, and to spread throughout the system.
Viruses cause varying degrees of harm.
Even seemingly innocuous viruses, though, can cause problems or unpredictable behavior in application programs. If a virus is using memory or disk resources that a legitimate programs needs, for example, the legitimate program might not operate properly. How Computer Viruses Spread Computer Viruses move from computer to computer when people share executable programs and bootable disks, either by disk swapping or over computer networks. In our world of on-line computer services, networks, shared computer facilities, and overnight mail services, computer viruses can spread at an alarming rate. Our experience is that many people are unaware that their computers have been infected by a computer virus, and attribute their problems to another cause.