Lassitude: A coordination system to support host immunity

Infectious disease is a powerful force of selection. Perpetual exposure to pathogens throughout evolutionary history has driven the evolution of multiple mechanisms that promote host survival in the face of disease. The highly conserved innate immune system is deployed in response to a wide variety of pathologies, including infection, injury, and toxin exposure. Activation of the innate immune system triggers a coordinated set of neuropsychological changes that help the host mount an effective immune response. These regulatory changes generate the experience of feeling sick. The authors of this chapter use the term lassitude to describe the superordinate coordinating program (i.e., emotion) that coordinates motivational and behavioral responses to disease. Lassitude is triggered by a reliably occurring situation (i.e., disease) and it orchestrates other mechanisms (e.g., appetite, fatigue, nausea, pain, and thermoregulatory motivation) to help solve the adaptive problems that arise when the innate immune response is activated to fight disease.