The author of the cited paper focuses on how the immune system can be emulated for fighting internet spam.
New infections stimulate the creation of T cells, or T lymphocytes, specific to the organism that set off the infection. If that organism appears again, the immune system is ready to attack. One problem with this method in the immune system is that the offending organism can avoid attack by changing its identity. Biological control relies on diversity. The more species there are, the more stable any ecological system is and the more resistant it is to attack from outside organisms. Response to parasites, viruses, bacteria, and other invaders lies in creating and maintaining diversity, and accepting some degree of damage. (summarized from Stevens 1998:64)