The clinical course of infectious diseases depends on the complicated interaction between the microorganism and the immune system of the host. The vast amount of knowledge acquired during recent years in the field of immunology will probably lead to new ways of treating infectious diseases by selective immunomodulation. In this paper, we review some newer aspects of host defence mechanisms of relevance to immunomodulation of infectious diseases. The cytokine network, apoptosis, superantigens, and oxidative stress are all discussed with reference to various infectious diseases. Furthermore, the possible role of physical and psychological stress in the immunopathogenesis of infectious disease is discussed in the light of the interplay between the immune system and the neuroendocrine system.