virus
n.
1. a microscopic parasitic agent that consists of an RNA or DNA core surrounded by a protective protein coat. Because viruses cannot replicate on their own but must invade a living host cell in an organism to do so, they are generally considered nonliving. Viral infection is responsible for many human illnesses and diseases, including influenza, poliomyelitis, mumps, several forms of cancer, and AIDS (see HIV). 2. a computer program designed to disrupt the functioning of other programs or to scramble or destroy computer files. It gains access to programs or systems surreptitiously, often by means of an attachment to an innocent-looking e-mail message. Viruses are typically designed to spread from computer to computer; for that reason, they can be extremely destructive.