A virus is a program that attaches itself to a program or file.

Acomputer virusismalicious software (malware)that replicates itself by attaching to legitimateprograms or files. Once activated, viruses can:

Corrupt or delete files.

Spread across networks, infecting multiple systems.

Cause performance issuesor even render a system inoperable.

Unlikeworms, which can spread independently, viruses requirehuman action(e.g., opening an infected file or running an executable).

References:McAfee Institute CCII Cyber Threat Guide, Cybercrime Encyclopedia​.