One of the most useful definitions of stress is as follows: Stress is an internal process that occurs when a person is faced with a demand that is perceived to exceed the resources available to effectively respond to it, and where failure to effectively deal with the demand has important undesirable consequences. In other words, stress is experienced when there is an awareness of a substantial imbalance between demand and capability, under conditions where failure to meet the demand is perceived to have unwanted consequences.

Strain is the negative effects of stress. Strain may appear as fatigue, irritability, difficulty concentrating, medical and physical problems, insomnia, depression, anxiety, over eating, drug and alcohol abuse, risk taking, or diminished functioning, to name a few of the possibilities. Stress can be positive and negative. On the positive side it alerts us to a threat and increases our level of arousal and activation which can help us be more effective in coping with the threat. It is mismanaged stress or an over-abundance of stress which causes strain and can be devastating for the person or the system.

Stress and health are closely linked. It is well known that stress, either quick or constant, can induce risky body-mind disorders. Research shows that in certain individuals stress does contribute to high blood pressure, high cholesterol, and other cardiac risk factors such as addictions and obesity.

The stress response of the body is meant to protect and support us. When faced with a threat, the body's defenses kick into high gear in a process known as the "fight or flight” response. The sympathetic nervous system pumps out adrenaline, preparing us for emergency action. Our heart rate and blood flow to the large muscles increase, the blood vessels under the skin constrict to prevent blood loss in case of injury, the pupils dilate so we can see better, and our blood sugar ramps up, giving us an energy boost.
The problem with the stress response is that the more it is activated, the harder it is to shut off. Instead of leveling off once the crisis has passed, your stress hormones, heart rate, and blood pressure remain elevated. With shear stress the body get adopted to the the response, and would not react to even other body metabolism activities, which stunts the growth process. Extended or repeated activation of the stress response takes a heavy toll on the body. The physical wear and tear it causes includes damage to the cardiovascular system and immune system suppression. Stress compromises your ability to fight off disease and infection, makes it difficult to conceive a baby, and stunts growth in children. It can even rewire the brain, leaving you more vulnerable to everyday pressures and mental health problems such as anxiety and depression. And, of course, the stress of living with a debilitating disease or disorder just adds to the problem.
Recent research suggests that anywhere from two-thirds to 90 percent of illness is stress-related.