Variation of susceptibility to infectious diseases has successfully been used to identify loci modulating response to specific diseases (Vögeli et al. 1999). However, since major health problems in intensive as well as animal welfare friendly production systems arise from multifactorial diseases, there is a need for animals with optimal endogenous protection against many pathogens including intra- or extra-cellular microorganisms. The selection of pigs for high and low immune responsiveness, using an index of five immune parameters of acquired and innate humoral and cellular mechanisms, showed improvement of protection and of responsiveness to numerous pathogens and of growth performance (Mallard et al. 1992). Identification of genes or markers linked to genes for disease resistance and immune competence facilitates the genetic improvement of animal health. We applied candidate gene and mapping approaches as well as global expression analyses in order to examine the emergence and inheritance of traits related to animal health and defence against infectious diseases.