Boot Sector Virus
Definition - What does Boot Sector Virus mean?
A boot sector virus is a computer virus that infects a storage device's master boot record (MBR). It is not mandatory that a boot sector virus successfully boot the victim's PC to infect it. As a result, even non-bootable media can trigger the spread of boot sector viruses. These viruses copy their infected code either to the floppy disk's boot sector or to the hard disk's partition table. During start-up, the virus gets loaded to the computer's memory. As soon as the virus is saved to the memory, it infects the non-infected disks used by the system.
The propagation of boot sector viruses has become very rare since the decline of floppy disks. Also, present-day operating systems include boot-sector safeguards that make it difficult for boot sector viruses to infect them.
Techopedia explains Boot Sector Virus
A boot sector virus usually infects the computer by altering the boot sector program. The virus replaces the default program with its own corrupted version. A boot sector virus is able to infect a computer only if the virus is used to boot up the computer. The computer will not be infected if the virus is introduced after the boot-up process or when the computer is running the OS.