The results of a virus can range from merely annoying to disastrous, and the loss of software and data can be time-consuming and expensive. It is a program designed to replicate and spread while the user is oblivious to its existence. When an infected file is executed, or when the computer is started from an infected disk, the virus is also executed. A virus often lurks in computer memory waiting to infect the next activated program or the next accessed disk. Viruses can rename system files so they cannot be accessed and make systems unbootable.
Viruses can spread by almost any means two computers share information.
The old-fashioned way, through an infected disk, is the most common. Most often, diskettes brought to the office from home are responsible for introducing a virus into a corporate network-computing environment. Most at-risk servers on a network have software that detects incoming viruses, but infections do occur in networked environments, making the rapid spread of a virus a serious risk. With networking, enterprise computing, and inter-organizational communications on the increase, infection during telecommunications and networking is on the rise. You can acquire viruses while downloading software from the Internet or from an electronic bulletin board service.