Stress causes illness by impairing the workings of the immune system, which guards the body against attackers. Psychoneuroimmunology studies the links between the mind, brain, and immune system, focusing on the effects of stress. The immune system works through white blood cells (leucocytes) that identify and destroy foreign bodies.

Stress affects natural killer cell cytotoxicity, which is crucial for defence against infections and cancer. Reduced levels of this cytotoxicity are found in highly stressed individuals, such as students facing exams, bereaved persons, and those severely depressed. Changes in the immune system have a more severe effect on health among those whose immune systems are already weakened.

Psychological stress is accompanied by negative emotions like depression, hostility, and anger. These negative moods are related to the function of the immune system and have been associated with poorer health outcomes. The sequence involves negative emotions leading to the release of stress hormones, which weaken the immune system, thereby affecting mental and physical health.

Stressed individuals are more likely to expose themselves to pathogens due to poor nutritional habits, lack of sleep, and health-risking behaviours like smoking and alcohol abuse. Stress produces changes in the immune system, increasing the chances of illness, and has been implicated in cardiovascular disorders, high blood pressure, and psychosomatic disorders. Researchers estimate that stress plays a role in fifty to seventy per cent of all physical illnesses.