They probably couldn't understand his letter. It is hard to understand why certain information was included, and other material was not. No Mac viri are described or listed although there is one example each from the Atari and Amiga environments. A specific recommendation is instructive. On an MS-DOS system, all .EXE files are to be renamed to .XXX extensions.
Errors are legion.
In some places the book contradicts itself, warning against BBSes and shareware on page 129 and yet saying that the danger of receiving viri from data transfer is no higher than through other means on page 292. It would be pointless to try to list them all, but I would be willing to bet that there are not three consecutive pages in the book which do not contain errors of fact. Chapter 5 is supposed to give examples of viral programs. Of the virus description material that Burger wrote, the only entries which do not contain errors are those which don't contain any information. He examines Fred Cohen's calculations in support of the assertion that a virus could not appear spontaneously by a generation from random errors.