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virus

Definitions:

  A computer program that can copy itself and infect a computer without permission or knowledge of the user. A virus might corrupt or delete data on a computer, use e-mail programs to spread itself to other computers, or even erase everything on a hard disk. See malicious code.
Sources:
CNSSI 4009-2015
NIST SP 800-12 Rev. 1 under Virus from CNSSI 4009

  A program that replicates itself by attaching to other programs or files, where it hides until activated.
Sources:
NIST SP 800-28 Version 2 under Virus

  A hidden, self-replicating section of computer software, usually malicious logic, that propagates by infecting (i.e., inserting a copy of itself into and becoming part of) another program. A virus cannot run by itself; it requires that its host program be run to make the virus active.
Sources:
NIST SP 800-82r3 from RFC 4949 - adapted