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.
Boot sectors exists on storage media, such as hard drives, floppies, or occasionally CDs and DVDs. During the booting process, the boot sector program is automatically located by the hardware and is then loaded. After this, the boot sector loads the remaining portion of the operating system into the memory. It is impossible for a computer to load the OS without a boot sector.
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.