Boot Sector Viruses

Until the mid-1990s, boot sector viruses were the most prevalent virus types, spreading primarily in the 16-bit DOS world via floppy disk. Boot sector viruses infect the boot sectors on floppy disks and hard disks, and can also infect the master boot record (MBR) on a user's hard drive. Once the machine is infected, it attempts to infect every floppy disk that is inserted into the computer and accessed.

Boot sector viruses work like this: by hiding on the disk, the virus is loaded into the memory before system files are loaded. This allows it to gain complete control of DOS interrupts so that it can spread and cause damage.