Hosts and microbes associate in a variety of relations along a continuum ranging from symbiotic to pathogenic. Defence mechanisms have been evolutionarily selected in both hosts and microbes to protect the organism's integrity. Such defences have to be utilized with caution. They must be adapted to the tasks at hand; otherwise any symbiotic relation would be impossible. To explain this cautionary use of defences we need to understand how life on Earth evolved into cooperative and competing entities at various levels of organization. The purpose of this article is to review theory and selected mechanisms relating to the evolution and development of host-microbe interactions, with special emphasis on host responses. The rationale is that without theory, extrapolations from misleading observations can dominate and distort, for a significant time, the course of a scientific field. The argument is set forth that social evolution theory provides a conceptual framework for addressing questions relating to interaction between hosts and microbes. The article is a partial summary of arguments presented in my book Defending life - the nature of host-parasite relations.