virus
A virus is a type of malware that attaches itself to legitimate programs or files and spreads from one system to another, typically causing harm by corrupting data, stealing sensitive information, or disrupting normal operations. It is called a “virus” because, like a biological virus, it infects systems, replicates, and spreads to other systems or programs.
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It "infects" other legitimate executable files or documents by attaching its code to them.
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When the user runs the legitimate program, they unknowingly execute the virus code first.