The Free On-line Dictionary of Computing (30 December 2018):

boot virus

   An MS-DOS virus that infects the boot record program on
   hard disks and floppy disks or the master boot record on
   hard disks.  The virus gets loaded into memory before MS-DOS
   and takes control of the computer, infecting any floppy disks
   subsequently accessed.  An infected boot disk may stop the
   computer starting up at all.

   (1995-02-16)