Final answer:
Boot-sector viruses load from USB drives left connected to computers during startup and infect the boot sector, allowing them to control the computer and spread to other drives.
Step-by-step explanation:
Viruses that load from USB drives left connected to computers when computers are turned on are known as boot-sector viruses. These viruses infect the boot sector of a storage device, which is the part of the drive that contains the code needed to start the boot process. When the computer is turned on, the virus is loaded into the computer's memory before the operating system, giving it control over the computer and potentially allowing it to spread to other bootable drives. These viruses were more common when floppy drives were standard, but they can still pose a threat with USB drives and other bootable media.