When a person experiences stress, the brain responds by initiating 1400 different responses including the dumping of a variety of chemicals to our blood stream. This gives a momentary boost to do whatever needs to be done to survive. If left unchecked, however, the person can have a heart attack or stroke. Many people start drinking alcohol. They get depressed, find it difficult to sleep, experience chest pain. The body runs out of the immunity to fight diseases. Doctors call the body's reaction to stress as General Adaptation Syndrome (GAS). There are three stages to GAS. 

In the first stage of GAS called alarm reaction, the body releases adrenaline and a variety of other psychological mechanisms to combat the stress and to stay in control. This is called fight or flight response. The muscles tense, the heart beats faster, the breathing and perspiration increases, the eyes dilate, the stomach may clench. Believe it or not, this is done by nature to protect you in case something bad happens. Once the cause of the stress is removed, the body will go back to normal. 
If the cause for the stress is not removed, GAS goes to its second stage called resistance or adaptation. This is the body's response to long term protection. It secretes further hormones that increase blood sugar levels to sustain energy and raise blood pressure. The adrenal cortex (outer covering) produces hormones called corticosteroids for this resistance reaction. Overuse by the body's defence mechanism in this phase eventually leads to disease. If this adaptation phase continues for a prolonged period of time without periods of relaxation and rest to counterbalance the stress response, sufferers become prone to fatigue, concentration lapses, irritability and lethargy as the effort to sustain arousal slides into negative stress.

The third stage of GAS is called exhaustion. In this stage, the body has run out of its reserve of body energy and immunity. Mental, physical and emotional resources suffer heavily. The body experiences "adrenal exhaustion". The blood sugar levels decrease as the adrenals become depleted, leading to decreased stress tolerance, progressive mental and physical exhaustion, illness and collapse. 

The hypothalamus-pituitary-adrenal (HPA) chain of command has served humans well as a means of survival for thousands of years. However, for those suffering from chronic anxiety and depression this process malfunctions. Continual stress early in life disrupts the cycle. Instead of shutting off once the crisis is over, the process continues, with the hypothalamus continuing to signal the adrenals to produce cortisol. This increased cortisol production exhausts the stress mechanism, leading to fatigue and depression. Cortisol also interferes with serotonin activity, furthering the depressive effect.

Continually high cortisol levels lead to suppression of the immune system through increased production of interleukin-6, an immune-system messenger. This coincides with research findings indicating that stress and depression have a negative effect on the immune system. Reduced immunity makes the body more susceptible to everything from cold and flu to cancer. For example, the incidence of serious illness, including cancer, is significantly higher among people who have suffered the death of a spouse in the previous year. Fortunately, this immune-suppression process can be corrected with psychotherapy, medication, or any number of other positive influences that restore hope and a feeling of self-esteem. The ability of human beings to recover from adversity is remarkable.

Thus, very often, those under severe, prolonged stress may contract diseases related to immune deficiency and may even die of these diseases. The death does not come from stress itself. What happens is that the body loses all its resistance in its effort to ward off the stress. Thus the persons die of immune deficiency causes such as infection, cancer etc. So, it is very important that we recognise the cause for stresses and remove the causes to maintain a healthy lifestyle.

Another result of stress is the clogging of the arteries by the fat and cholesterol released by the body during the attempt to fight stress. This may result in a heart attack or you may suffer a stroke by losing blood supply to the brain. Many people start drinking to combat the stress. Stress can also manifest itself into a number of diseases - depression, headaches, insomnia, ulcers, asthma, and more.