Stress, in regards to the body, can be defined as anything perceived or occurring which triggers the sympathetic nervous system to react to it; a threat or challenge, whether it be physical or mental. A stressor is whatever is causing the stress response.
The ability to deal with stress or stressors is a major determinant of your capability to maintain wellness.
A certain degree of positive, controlled stress can be good for you (see Stress Curve diagram). It can stimulate motivation, relieve laziness and boredom, enhance the ability to cope / adapt in times of adversity, and it can also stimulate the reward system of the brain triggering the production of feel-good neurotransmitters when goals are achieved.
So why is stress described as "the silent killer" here? Because many people are unaware that they are in a predominantly, continual state of sympathetic nervous system (SNS) mode (fight or flight mode). They have become so accustomed to stress just being a part of their life, due to the pressure they place on themselves, and/or external pressures of the fast-paced, continually goal orientated society that many people live in, not understanding why they are prone to getting sick, taking visits to the doctor for short-term, symptomatic relief so that they can carry on with their stressful lifestyle as best they can, but not actually addressing what would have to be one of the greatest causes of ill-health, STRESS. Basically not seeing the elephant in the room until it squashes them!!
Your body is designed to go through physiological changes in response to stressors, so as to enhance its performance in order to be able to respond (fight or flee from) a perceived stress, and ultimately survive the ordeal... fulfillment of your innate survival instinct. Once the threat (stressor) has disappeared, your body is also designed to move back into parasympathetic nervous system (PNS) mode, a more relaxed mode, which enables it to return to a state of balance and normal functioning of body systems (homeostasis).
This equilibrium between survival and relaxation goes way back in our DNA programming... the cave man having an encounter with a wild, ferocious animal, the subsequent trigger of the fight or flight response (SNS mode) in order to assist the cave man to survive the encounter (stressor), and once the animal has gone, the return to a more relaxed state (PNS mode), allowing the body to continue all the bodily functions that are not absolutely necessary for immediate survival, but are absolutely essential for maintaining optimal long-term wellness.
However, in modern society, stressors can come from many varied sources, other than just the decision to fight or flee from a wild animal; from extreme stressors (e.g avoidance of a life and death situation, extreme sports, pressure-loaded/fast-paced activities), to the more subtler forms of ongoing stress (e.g work demands, time deadlines, overload - too much going on at once, emotional or physical trauma).
We were designed to be able to deal with short-term stressors to enable immediate survival, but it is the longer-term, subtler, forms of stress that cause the body to be predominantly in flight or fight mode (SNS) on a long-term, on-going basis. If this form of stress (ie chronic stress) goes on for extended periods, it is inevitable that wear and tear will occur throughout the entire body due to sub-optimal functioning of body systems, particularly those that are not as necessary for immediate survival such as the digestive and immune / lymphatic system, leading to disarray of normal, healthy, physiological function.
Manifestation of the effects of over-active, long-term stress response can be more likely to occur in vulnerable systems or organs of the body; those that may be weaker for reasons such as genetic predisposition, past disease history, previous trauma sites; or it can manifest as a general disruption to biological functioning, causing lack of vitality, unhealthy immune response (under or over-reactive), cardiovascular damage, mental and emotional instability, endocrine system mal-function, urinary system abnormalities, and poor digestive processes potentially leading to nutrient deficiencies, leaky gut, IBS, constipation, diarrhoea e.t.c, which in turn can then hinder the function of other body systems and detoxification pathways, creating even greater disease susceptibility and ill-health.
Once in this lowered vitality state it only takes a major incident, or sometimes even a minor one depending on your level of lowered vitality, to instigate a major breakdown or system failure due to the body's already poor state of function, and therefore lack of inability to adapt effectively to any stressor whatsoever.
You can learn more in an upcoming article about ways to assist the body to be better equipped to respond to stressors, and therefore be better equipped to deal with the ups and downs of life. This can help reduce the detrimental effects that stress can have on your physical and mental health, and to ensure that the body is capable of returning to a state of equilibrium and healthy functioning once a stressor has passed.