In medical terms stress is described as, "a physical or psychological stimulus that can produce mental tension or physiological reactions that may lead to illness." When you are under stress, your adrenal gland releases corticosteroids, which are converted to cortisol in the blood stream. Cortisol have an immune suppressive effect in your body.
According to Richard S Lazarus, stress is a feeling experienced when a person thinks that "the demands exceed the personal and social resources the individual is able to mobilize." Stress is a term in psychology and biology, first coined in the biological context in the 1930s, which has in more recent decades become a commonplace of popular parlance. It refers to the consequence of the failure of an organism – human or animal – to respond appropriately to emotional or physical threats, whether actual or imagined.
Stress symptoms commonly include a state of alarm and adrenaline production, short-term resistance as a coping mechanism, and exhaustion, as well as irritability, muscular tension, inability to concentrate and a variety of physiological reactions such as headache and elevated heart rate.