Stress, a term commonly used to describe varied phenomena, should be restricted to describe an adaptive response by an animal to threats to homeostasis. The threats to homeostasis are called stressors. Stressors include a variety of physical, psychological, chemical, or infectious causes that are modified by intrinsic and extrinsic factors. Examples of modifiers include stressor severity, duration, novelty, host genetics and immune status. What may be a stressor to an animal in one situation, when modified, may not be a stressor in another situation. Mechanisms of stress once thought to involve a single pathway described by Seyle as the General Adaptation Syndrome, have been rejected. Four pathways, some incompletely defined, have been implicated in modulation of the immune system. They include autonomic nervous system, the hypothalamic adrenal axis, extra-adrenal pathways involving neuropeptides and neurotransmitters and neuroimmunological mediators. The mechanisms of stress-induced immunosuppression resistance are poorly defined in domestic fowl and will require careful experimentation linking defined stressors with altered physiological responses that affect specific immune function and result in increased disease susceptibility.