A pathogen may emerge as an important public health problem because of changes in itself or its transmission pathways. Alternatively, a microorganism may emerge as a pathogen or acquire new public health importance because of changes in host susceptibility to infection. Factors influencing host susceptibility within the population as a whole include increases in the number of immunocompromised patients; increased use of immunosuppressive agents, particularly among persons receiving cancer chemotherapy or undergoing organ transplantation; aging of the population; and malnutrition. In considering the emergence of foodborne pathogens and designing interventions to limit their spread, the susceptibility of these population subgroups to specific infections should be taken into account.