Sickness behavior are nonspecific behavioral changes humans and many animals adopt in response to an infection. Responses include lethargy, lack of appetite, low grade fever, sleepiness and lack of motivation. It is thought to have evolved to encourage behaviors that conserve energy and allow the host to better mount an immune response against invading pathogens.
In response to the presence of a pathogen, proinflammatory cytokines IL-1, IL-6 and tumor necrosis factor (TNF) are released peripherally. These may then be sensed by the vagus nerve initiating a set of behavioral responses in the brain.