What is Boot sector viruses?

Boot sector viruses

Boot sector viruses infect the boot record on hard disks, floppy disks, and theoretically also on CD's and DVD's.

A boot sector virus does not need to be able to successfully boot the victims computer to infect it. Because of this, even non-bootable media can spread a boot sector virus.

Once the infected computer does successfully boot, the boot sector virus stays in memory and infects floppies and other media when they are written to by the infected computer.
Boot sector viruses have become less common as floppy disks have become rarer.