Minute, nonliving infectious disease agents composed primarily of protein and nucleic acid (either RNA or DNA), and characterized by the absence of independent metabolism and an inability to replicate outside susceptible host cells.
noncellular, microscopic particles that replicate themselves within invaded cells.
In computer security technology, a virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents. A computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells. Extending the analogy, the insertion of the virus into a program is termed infection, and the infected file (or executable code that is not part of a file) is called a host. Viruses are one of the several types of malware or malicious software.