a portion of a program code that has been designed to furtively copy itself into other such codes or computer files. It is usually created by a prankster or vandal to effect a nonutilitarian result or to destroy data and program code.

A virus consists of a set of instructions that attaches itself to other computer programs, usually in the computer's operating system, and becomes part of them. In most cases, the corrupted programs continue to perform their intended functions but surreptitiously execute the virus's instructions as well. A virus is usually designed to execute when it is loaded into a computer's memory. Upon execution, the virus instructs its host program to copy the viral code into, or “infect,” any number of other programs and files stored in the computer. The infection can then transfer itself to files and code on other computers through magnetic disks or other memory-storage devices, computer networks, or online systems. The replicating viruses often multiply until they destroy data or render other program codes meaningless. A virus may simply cause a harmless joke or cryptic message to appear on a computer user's video monitor each time he turns on his computer. A more damaging virus can wreak havoc on an extremely large computer system within a matter of minutes or hours, causing it to crash and thereby destroy valuable data.