noun. 1. a microscopic parasitic compound which contains an RNA or DNA center encompassed by a safeguarding protein coating. As viruses can't multiply or reproduce without invading a living host cell in a living being to do so, they are usually thought to be non-living. Viral infection is accountable for a multitude of human diseases and illnesses, inclusive of mumps, the flu, AIDS, poliomyelitis, and various types of cancer. 2. a computer program modeled to interrupt the operating of other programs or to confuse or disassemble computer files. It gains entry to programs surreptitiously, frequently by way of an attachment to a seemingly problem-free e-mail. Viruses are commonly made to spread from computer to computer; for this reason, they can be very destructive.