This basic security principle is frequently ignored and this can result in disaster. Diversity is good. Though the virus picked on the most widespread operating system used on the Internet and on the two most popular machine types, most of the machines on the network were never in danger. A wider variety of implementations is probably good, not bad. There is a direct analogy with biological genetic diversity to be made.
Backups are good.
Defenses must be at the host level, not the network level. Attempts to fix the network are misguided. Logging information is important. The inetd and telnetd interaction logging the source of virus attacks turned out to be a lucky break, but even so many sites did not have enough logging information available to identify the source or times of infection. This greatly hindered the responses, since people frequently had to install new programs which logged more information.