Stress generally refers to two things: the psychological perception of pressure, on the one hand, and the body's response to it, on the other, which involves multiple systems, from metabolism to muscles to memory.
Some stress is necessary for all living systems; it is the means by which they encounter and respond to the challenges and uncertainties of existence. The perception of danger sets off an automatic response system, known as the fight-or-flight response, that, activated through hormonal signals, prepares an animal to meet a threat or flee from it.
A stressful event—whether an external phenomenon like the sudden appearance of a snake on the path or an internal event like fear of losing one's job when the boss yells—triggers a cascade of hormones, including adrenaline and cortisol, that surge through the entire body, speeding heartbeat and the circulation of blood to support quick action, mobilizing fat and sugar for fast energy, focusing attention to track the danger, preparing muscles for movement, and more.
Lifesaving as the stress response is, it was meant to solve short-term, life-threatening problems, not extended difficulties such as daily traffic jams or marital problems. It generally takes some time for the body to calm down after the stress response has been triggered. Prolonged or repeated arousal of the stress response, a characteristic of modern life, can have harmful physical and psychological consequences, including heart disease and depression.