Events that cause stress are known as stressors, this covers a wide range of situations. Stressors can be anything from risking physical danger to taking a test in class or even giving a presentation at work. The human body responds to these stressors by activating the nervous system and specific hormones such as adrenaline and cortisol. The adrenal glands receive the signal from the hypothalamus to produce more of these hormones and then release them into the bloodstream. The hormones adrenaline and cortisol increase breathing rate, heart rate, blood pressure and metabolism. The blood vessels then dilate to allow more blood flow to reach large muscle groups; the pupils also dilate to improve vision. The liver then releases stored glucose to increase the body's energy, then sweat is produced to cool the body (Neil et al., 2007). All physical changes prepare the body to react quickly and effectively so that the pressure of the moment can be handled in a controlled manner. This natural reaction is known as the stress response. The body's stress response can improve a person's ability to perform well under pressure if it functions properly, however, the stress response can also be dangerous, causing serious problems if it overreacts or fails to recover properly. Many physiologists today also define stress as the body's reaction to a stressor. Acute stressors affect an organism in the short term, while chronic stressors affect an organism for a longer period. Hans Hugo Bruno Selye, the Canadian endocrinologist, studied the effects of stress and found that stress differs from other physical responses. He believed that stress is a stressful experience regardless of whether one receives good or bad news and also whether the impulse is positive or negative. He defined negative stress as distress and positive stress as eustress (Obama et al., 2007). In 1936, Selye studied a stress model known as general adaptation syndrome (GAS) where he experimented on rats by injecting them with extracts of various organs. The rat experiment can show whether the organism is seriously damaged by acute non-specific harmful agents, examples include exposure to cold, production or transcision of the cord (spinal shock), surgical injuries, excessive exercise and intoxications from lethal drugs such as morphine, adrenaline, atropine and formaldehyde (Selye, 1998). Throughout his research, Selye described the effects of stress in three key stages; These stages explain how the human body can become susceptible to infections and diseases.