When we have a flu or an infection or we are stressed, our immune cells produce molecules (pro-inflammatory cytokines) to help us fight if off. While these signaling molecules activate our immune system, they also act upon the brain, making us feel sick. When our immune systems continues to be activated for long periods of time, such as occurs during chronic stress, the effects of cytokines on the brain can worsen this feeling of sickness and lead to the symptoms of depression. Inflammation may be an important way in which life's overwhelming events and unresolvable stresses culminate in depression.
Anyone who has experienced a viral or bacterial infection knows what it means to feel sick. Sick people often feel feverish and nauseated, ignore food and beverages, and lose interest in their surroundings and in other people. They tire easily and their sleep is often disturbed. They lose interest in activities and become irritable, and can experience difficulty thinking, including problems with attention and memory. When we are acutely ill, we usually try to ignore these symptoms, knowing that our illness will pass and the symptoms will disappear
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