Revision with unchanged content. Today's computer systems are running (mostly) one kind of operating system which makes it easy for a new virus to spread rapidly. The author Markus Unterleitner presents in this book the model and the implementation of the computer immune system (CIS) based on several mechanisms of the human immune system. It is shown how effective the implementation of the CIS is at detecting malicious code in packets of the network traffic. The implemented CIS is not centrally or hierarchically controlled and it is intended to shield computers in a LAN from new network driven attack attempts. The targets for this adaptive and distributed detection system are Internet worms, different kinds of viruses and shellcodes, which are possibly polymorph. Biological inspired mechanisms make it possible that the computer systems have independently evolving shields. This makes systems more diverse and they become more robust to today's common and zeroday threats. This book addresses people who are entrusted with or interested in network security. It goes also to researchers or students in the computer security field and people who are maintaining or protecting computer networks in companies.