This reminds me of a passage in The Andromeda Strain. I don't have the book here to do an exact quote, but it mentioned that, just as a person easily killed by

This reminds me of a passage in The Andromeda Strain. I don't have the book here to do an exact quote, but it mentioned that, just as a person easily killed by bacteria was a failure (in that they wouldn't live long enough to reproduce), a bacterium that killed its host was also a failure, because it must die when the host dies. The most successful bacteria are those that can live off a host without killing them, and the most successful hosts are those that can tolerate the presence of bacteria, or even make them work for the host.