Taken from the obvious analogy with biological viruses, a computer “virus” is a name for a type of computer program that searches out uninfected computers, “infects” them by causing them to execute the virus, and then attempts to spread to other computers. A virus does two things: execute code on a computer, and spread to other computers.

The executed code can accomplish anything that a regular computer program can do; it can delete files, send emails, install programs, and can copy information from one place to another. These actions can happen immediately or after some set delay. It is often not noticed that a virus has infected a computer because it will mimic the actual actions of the infected computer. By the time it is recognized that the computer is infected, much damage may have occurred.

Earliest computer viruses spread via physical media, such as floppy disks. Modern viruses propagate rapidly throughout the Internet. In May 2000, the “ILOVEYOU” virus made international headlines as it spread around the world in a single day, crashing millions of computers and costing approximately $5 billion in economic damages. Since then, criminals have learned to prevent their viruses from crashing computers, making detection of these viruses much more difficult. The highly advanced Conficker worm, first detected in 2008, operates silently, ensuring that the typical computer user won’t realize that the virus is present.