People possess a regulatory program that lies in wait, scanning for indicators that infectious disease is present. If it detects signs of infection: 

• In order to avoid additional infections or injuries that would increase the immune system's energy requirements, your body increases sensitivity to nausea and pain.
• Sensitivity to cold motivates you to find warm clothing and heat sources that reduce the costs of keeping you...

Illness comes with a distinct set of psychological and behavioral features. You feel fatigued, maybe a little depressed, less hungry than usual, more easily nauseated and perhaps more sensitive to pain and cold.