One of the original forms of viruses, is usually stored on some form of removable media. When the removable media is connected to a new machine and is used to boot the machine, the virus moves from the media into the host system. Newer versions do not need to be booted; they simply have to reside on the removable media, such as on a flash drive.

Boot sector virus

Explanation:

A boot sector virus is a virus which functions is to infects the sector of the boot  of floppy disks of hard disks in which they use DOS commands to spread to other floppy disks and they tend to  either copies the master boot program to another part of the hard disk or try to overwrites it which is why they are said to always infect a computer when it boots up as well as when it accesses the infected floppy disk in the floppy drive. Although BOOT SECTOR VIRUS are difficult to remove because most antivirus programs cannot clean the MBR while the windows is running reason been that it takes bootable  antivirus disks to properly remove a boot sector virus.

Hence with BOOT SECTOR VIRUS newer versions do not need to be booted due to the fact that they often have to reside on the removable media, such as on a flash drive especially in a case when the Computer systems are typically infected by these viruses.