A noncellular biological entity that can reproduce only within a host cell. Viruses consist of nucleic acid covered by protein; some animal viruses are also surrounded by membrane. Inside the infected cell, the virus uses the synthetic capability of the host to produce progeny virus. A computer program designed to covertly infiltrate and usually damage a computer system, either by repeatedly copying itself so that it takes up needed memory or by erasing, modifying or damaging other files.