This paper is based on examples from experiments in sheep conducted by pathologists and geneticists of INRA about salmonellosis, scrapie and gastro-intestinal strongylosis. In some cases, resistant animals are carriers of pathogens and could act as sources of infection to others. Another important aspect is the specificity of resistance. In an animal infected by gastro-intestinal nematodes, three main concepts are to be considered: (i) resistance which controls the size of worm burden; (ii) resilience, i.e., ability for an animal to maintain its production when infected; and (iii) contamination power related to the number of eggs excreted on pasture. Evaluation of resistance may be based on either direct or indirect criteria. Direct criteria are recorded either under field conditions or during a challenge with pathogen. They are related to numbers of pathogens in target organs, clinical signs, pathogenic effects (e.g., anaemia in haemonchosis), host immune response, etc. Indirect criteria are potential predictors independent of exposure to the pathogen (humoral response to a specific or non-specific challenge, genetic markers, etc.). Some examples of genetic determinism are given: major loci (Prn-p in scrapie and possibly Nramp in salmonellosis) or polygenes (gastro-intestinal parasitism). Identification of QTL (Quantitative Trait Loci) becomes feasible with the progress in genome mapping. Breeding strategies have to be set up. Inclusion of an additional selection criterion concerning resistance in a pre-existing breeding programme requires the objective to be defined accurately and to know, not only the genetic variability of the resistance trait, but also its genetic relationships with production traits.