The transfer process of virus is much sophisticated today. Earlier, it was pretty simple! Business people who traveled the world with floppy disks in their briefcases could infect the computers on which they used the disks. BRAIN was the first such virus to be made. It exploited a security issue in MS-DOS, the OS prior to the advent of Windows, by replacing the boot sector of the disk with the virus and renaming it with the name
BRAIN. All subsequent disks placed in the drive would then be infected.Full Article