Stress can be defined as a state of worry or mental tension caused by a difficult situation. Stress is a natural human response that prompts us to address challenges and threats in our lives. Everyone experiences stress to some degree. The way we respond to stress, however, makes a big difference to our overall well-being.
(Source: World Health Organisation)

Stress is a feeling of emotional strain and pressure. Stress is a type of psychological pain. Small amounts of stress may be beneficial, as it can improve athletic performance, motivation and reaction to the environment. Excessive amounts of stress, however, can increase the risk of strokes, heart attacks, ulcers, and mental illnesses such as depression and also aggravation of a pre-existing condition.

Hans Selye, the father of stress, proposed four variations of stress (1975):

• On one axis he locates good stress (eustress) and bad stress (distress). The term "eustress" comes from the Greek root eu- which means "good" (as in "euphoria").Eustress results when a person perceives a stressor as positive."Distress" stems from the Latin root dis- (as in "dissonance" or "disagreement"). Medically defined distress is a threat to the quality of life. It occurs when demands vastly exceeds a person's capabilities.
• On the other is over-stress (hyperstress) and understress (hypostress).

Selye advocates balancing these: the ultimate goal would be to balance hyperstress and hypostress perfectly and have as much eustress as possible.

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Hans Selye's theory of “General Adaption Syndrome” suggests stress plays a role in every disease. There are three stages in the general adaptation syndrome: 

Stage 1: ALARM - when we perceive a threat and the body activates fight, flight or freeze to respond to the stressor. 
​This involves hormones and chemicals excreted at high levels as well as an increase in heart rate, blood pressure, perspiration, respiration rate, etc.

Stage 2: Resistance - the body becomes adaptive to the challenge and attempts to cope or adapt to the stressor.
The length of this stage depends on the body’s innate and stored adaptation energy reserves and the intensity of the stressor. 

Stage 3: Exhaustion - when the stressor persists and our resources are too depleted to cope with the demands. This leads to physical, mental and emotional health problems, eventually to death.
Stress diseases are maladies caused principally by errors in the body’s general adaptation process. They will not develop when adaptation is facilitated by improved perception and interpretation. Source: The American Institute of Stress 

HOW ​Stress Affects Your Body and Mind

Stress is a natural physical and mental reaction to life experiences — everyone experiences it. Anything from everyday responsibilities like feeding your family to more serious life events like a car accident can trigger stress. In immediate, short-term situations, stress can benefit your health. It can help you cope with potentially serious situations.
Your body responds to stress by releasing hormones that increase your heart and breathing rates and ready your muscles to respond; however, if this response continues longer than necessary for survival, it can take a toll on your health.
Chronic stress can cause even more symptoms affecting your overall well-being, such as irritability, anxiety, depression, headaches, or insomnia

Central Nervous & Endocrine Systems
The central nervous system (CNS) controls your “fight or flight” response: the hypothalamus tells your adrenal glands to release the stress hormones - adrenaline and cortisol. When the perceived fear is gone, the hypothalamus should tell all systems to return to normal. If the CNS fails to normalize or the stressor remains, then the response continues.
Respiratory & Cardiovascular Systems
Stress hormones make you breathe faster to quickly distribute oxygen-rich blood to your body. Stress hormones make your heart pump faster and cause your blood vessels to constrict and divert more oxygen to your muscles, so you’ll have more strength to take action. It also raises your blood pressure. If the heart works too hard for too long, the risk for heart attack and stroke increases.
Digestive System
Your liver produces extra blood sugar to give you more energy. Under chronic stress, you risk to develop type 2 diabetes. Stress increases the risk for ulcers and acid reflux. It leads to diarrhea or constipation, nausea, vomiting, or stomachache.
Muscular System
Under constant stress, muscles may not get the chance to relax. Tight muscles cause headaches, back and shoulder pain, and body aches. 
Sexuality & Reproductive System
While short-term stress may cause men to produce more of the male hormone testosterone, long-term stress makes testosterone levels drop, causes erectile dysfunction or impotence, increases risk of infection for male reproductive organs. For women, stress leads to irregular, heavier, or more painful periods. 
Immune System
Long stress will weaken the immune system.
Source: The American Institute of Stress 

​50 Common Signs and Symptoms of Stress

• Frequent headaches, jaw clenching, or pain
• Gritting or grinding teeth
• Stuttering or stammering
• Tremors or trembling of lips/hands
• Neck ache, back pain, muscle spasms
• Light headedness, faintness, dizziness
• Ringing, buzzing or popping sounds
• Frequent blushing or sweating
• Cold or sweaty hands and feet
• Dry mouth, problems swallowing
• Frequent colds, infections, herpes sores
• Rashes, itching, hives, goose bumps
• Unexplained or frequent allergy attacks
• Heartburn, stomach pain, nausea
• Excess belching, flatulence
• Constipation, diarrhea, loss of control
• Difficulty breathing, frequent sighing
• Sudden attacks of life- threatening panic
• Chest pain, palpitations, rapid pulse
• Frequent urination
• Diminished sexual desire or performance
• Excess anxiety, worry, guilt, nervousness
• Increased anger, frustration, hostility
• Depression, frequent or wild mood swings
• Increased or decreased appetite

• Insomnia, nightmares, disturbing dreams
• Difficulty concentrating, racing thoughts
• Trouble learning new information
• Forgetfulness, disorganization, confusion
• Difficulty in making decisions
• Feeling overloaded or overwhelmed
• Frequent crying spells or suicidal thoughts
• Feelings of loneliness or worthlessness
• Little interest in appearance, punctuality
• Nervous habits, fidgeting, feet tapping
• Increased frustration, irritability, edginess
• Overreaction to petty annoyances
• Increased number of minor accidents
• Obsessive or compulsive behavior
• Reduced work efficiency or productivity
• Lies or excuses to cover up poor work
• Rapid or mumbled speech
• Excessive defensiveness or suspiciousness
• Problems in communication or sharing
• Social withdrawal and isolation
• Constant fatigue and weakness
• Frequent use of nonprescription drugs
• Weight gain or loss without diet
• Increased smoking or drug/alcohol use
• Excessive gambling or impulse buying

Source: The American Institute of Stress ​