Final answer:
Viruses that load from USB drives during computer startup are known as boot-sector viruses. They infect the boot sector or master boot record and can take control over the boot process.
Step-by-step explanation:
The type of viruses that load from USB drives left connected to computers when the computers are turned on are known as boot-sector viruses. These viruses are specifically designed to infect the boot sector of a computer's hard drive or the master boot record (MBR) of removable devices like USB drives. When the infected device is connected to a computer during its startup process, the virus can execute and potentially take control over the boot process, leading to a successful infection of the host computer's system.
In comparison to other virus types, boot-sector viruses are less common today due to enhancements in both hardware and software security, though they were more prevalent in the past when floppy disks were commonly used to boot computers.