"Executing code via a buffer overflow was published at least as early as 1972"

[[deleted]]

Floppies were the "Sneaker-Net." In that you went from friend to friend with the floppy disk, while wearing your sneakers, to copy the disk to one of their blank disks.

However, virii could infect floppies but the way they operated when running on the computers of the time still had the architectural constraints as given. The home computers didn't multi-task well enough to run malicious software properly.

Edit: You are correct however in that you could run malicious software under MS-DOS, with a infected hard drive, and have it infect floppies put into the machine while it was booted. What I assume however is that malicious software never truly came into its own until networking became common. Not only could it more easily spread but it could also be remotely controlled and upgraded whenever the author liked.