Stress is a state of physical, mental or emotional strain. If not managed effectively it may lead to a wide array of health problems and can be a significant factor in many serious illnesses.
The hormonal, nervous and immune systems can all send information between each other directly and very quickly. Therefore, the physiological response to stress messages being sent from the brain can include increased secretion of adrenaline and cortisol, a rise in heart rate and blood pressure, muscular tension, digestive disorders, higher levels of cholesterol, nutritional deficiency, suppression of immune cells (ie antibodies and lymphocytes) and general changes in blood composition.
Modern psychological theory has identified that repeated behaviour can eventually become habitual. The brain builds up and stores a behaviour pattern so that the response to a particular situation starts to feel instinctual and operates like an underlying, core belief. Automatic (instinctual) responses or perceptions generate emotions. These emotions arise before conscious thoughts and the more emotional we are, the more difficult it is to think clearly, remain calm and make choices that will enhance well being.
This would all explain why many psychiatrists view stress as a common precursor of back problems, irritable bowel syndrome, chronic fatigue, anxiety and depression.