You would get a floppy disk that had a virus buried in it in addition to the game/software. Often via trade of copied software, I think there were one or two instances of officially released floppies having been compromised in the factory, too, so every floppy sold of that particular software came with the virus.

It’d infect your computer, then embed itself on every floppy you’d make, some of which you’d inevitably trade with friends or whatever, thus infecting all their systems, so all floppies they’d write were infected, and they’d eventually trade with others, and so on and so on and so on.