A virus written into the boot sectors of a floppy disk. A popular way to spread a virus when floppy disks were widely used, the boot virus relied on people forgetting to remove the last floppy they inserted when they turned the machine off. When turned back on, the machine read the boot sector program, which normally loads the operating system, but ran the infected program instead. Once infected, the boot virus replicated itself onto all subsequent floppies used in the machine.
The Michelangelo virus was a famous boot virus that infected DOS computers on March 6th, Michelangelo's birthday. However, there has always been doubt whether the date was really on account of the celebrated artist and sculptor. See virus