It would seem the authors know essentially nothing about computer systems and viruses. Ruins the book for anyone who has even a modicum of understanding of how they work.
To have the effects the book describes, there would have to be many different viruses, not one:- different ones for each operating system (Windows, Mac, LINUX, UNIX, mainframes, and so on). Really critical systems (e.g. military) often have NO connection to the internet, and so would be immune. The group trying to remove the virus would have been one such.