Stress is a non-specific response of the body to stressor which is perceived as a threat to homeostasis. Each stressor causes stress and a specific action. Manifestation of the disease depends on the stressors specific action, individuals internal factors (age, gender, breed, genetic inheritance) and external factors (nutrition, housing conditions, ect.). There are two types of stress, positive (eustress) and negative stress (distress). The stress response is a useful physiological response to stress and consists of three phases: alarm reaction, resistance phase and exhaustion phase. It is a combination of behavioral responses, responses of the autonomic nervous system, neuroendocrine and immune response. The result of stress response can be return to homeostasis or development of pathological conditions. Adrenal hormones and the sympathetic nervous system regulate stress response. Stress can suppress, potentiate or not have any effect on the immune system depending on the type of stressor (psychic, physical, etc.), on the genetic properties of the individual, age, social hierarchy and whether the effects are acute or chronic. If stress is prolonged, the immune system is suppressed through the hypothalamic-pituitary-adrenal gland axis. The influence of the genetic heritage as the intrinsic factor is used to reduce the sensitivity of animals to stress which consequently, results in faster growth, better reproductive capability and a reduced incidence of diseases