Boot sector viruses spread by modifying the program that enables your computer to start up.

When you switch on a computer, the hardware looks for the boot sector program, which is usually on the hard disk, but can be on a floppy disk or CD, and runs it. This program then loads the rest of the operating system into memory.

A boot sector virus replaces the original boot sector with its own, modified version (and usually hides the original somewhere else on the hard disk). When you next start up, the infected boot sector is used and the virus becomes active.

You can only become infected if you boot up your computer from an infected disk, e.g. a floppy disk that has an infected boot sector.

Boot sector viruses were the first type of virus to appear, and they are mostly quite old. They are rarely encountered today.