This article reviews research on the role of psychological stress, personality, social support and other psychosocial factors in bacterial, viral and parasitic infections. After 100 years of research on man and animals, psychological stress is considered as a potential cofactor in the pathogenesis of infectious disease. Psychological stress seems able to alter the susceptibility of animals and man to infectious agents, influencing the onset, course and outcome of certain infectious pathologies. Many experiments have identified in neuroimmunomodulation the principal mediator of the alterations associated with conditions of stress. The development of psychoneuroimmunology has fostered in-depth study of the complex relationship between psychosocial factors, the central nervous system, the immune system and infectious disease. Although antimicrobial drugs have certainly remained the basis of all anti-infective therapy, this type of study has already led some authors to propose and experiment protocols of psychological intervention or psychoimmunotherapy in pathologies such as tuberculosis, or herpes simplex virus or human immunodeficiency virus infections. The psychoneuroimmunological approach to infectious diseases will probably grow in importance in the future not only in the setting of research in psychosomatic medicine but also in that of clinical microbiology.