You probably don't interact with your PC's BIOS (Basic Input/Output Operating System) much, but it occupies a unique and highly privileged position in your computer's architecture.
Since the BIOS loads before the operating system - and before you enter your user credentials - malware surreptitiously introduced into the BIOS could activate itself long before any anti-malware software has an opportunity to detect it. A sophisticated and malicious program operating at such a low level could take control of your PC without providing a clue that it was there.

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