Know how to use "fewer" and "less"? Find out.
A program that enters a computer usually without the knowledge of the operator. Some viruses are mild and only cause messages to appear on the screen, but others are destructive and can wipe out the computer's memory or cause more severe damage.
Note: Computer viruses spread from machine to machine on disks or, more commonly, over the Internet.
Note: The creation of computer viruses can be part of cyberwarfare.