What makes chronic stress potentially lethal is its duration and the constant accumulation of its effects. Stress upon stress grows like compound interest on a loan. When only the minimum payment is made, the balance continues to grow and can never be fully repaid.
Humans respond to stressors such as physical or perceived danger, an infection, or a crowded and noisy environment, by initiating a complex biopsychosocial adaptation and coping response. This response is initiated by the sympathetic nervous system and leads to release of excitatory stress hormones (the catecholamines) and glucocorticoids from the adrenal cortex (the well-known adrenaline rush).
The objective is to engage with the situation, resolve it and return to the status quo. This process of regaining stability through change and adaptation is called allostasis. The arousal and mobilization of biopsychic resources is intended to be temporary and is shut off when the challenge has passed.