This was followed by Dark Avenger's Commander Bomber. Before CB, you could very easily predict where in the file the virus would be. Bomber transforms this, and so products either have to scan the entire file, or else they have to be more sophisticated about locating the virus. Another virus that came out at about that time, was Starship. Checksumming programs aim to detect a virus by the fact that it has to change executable code in order to replicate.
Starship only infects files as they are copied from the hard disk to the floppy.
So files on the hard disk never change. But the copy on the floppy disk is infected, and if you then copy that onto a new hard disk, and tell the checksummer on the new machine about this new file, the checksummer will happily accept it, and never report any changes. Starship also installs itself on the hard disk, but without changing executable code. It changes the partition data, making a new partition as the boot partition. No code is changed, but the new partition contains the virus code, and this is run before it passes control on to the original boot partition.