An extended concept of altered self: chronic fatigue and post-infection syndromes.

Sickness behavior in active infectious diseases is defined here as the responses to cytokines and other mediators of inflammation as well as the adaptability of a pre-existing integrated immunological, psychological, neurological, and philosophical self. These complex behaviors are biologically advantageous to the afflicted individual, but they also impact surrounding individuals. If chronic conditions, such as chronic fatigue syndrome or post-infection fatigue, exhibiting these behaviors follow infection in the absence of ongoing changes in immunological self associated with an active infection or subsequent injury, they are currently considered illness states rather than true diseases. Self-referential recognition (interoception) of bodily processes by the brain and subsequent unconscious and conscious adaptive responses arising in the brain, i.e., in the endocrine system and immune systems, which are initiated during the infection and would normally lead to positive maintenance, may become maladaptive and lead to an "extended altered self state." Exploratory measurements of such alterations using a "top-down" approach such as monitoring responses to appropriate challenges can be obtained using functional brain imaging techniques. Once identified, processes remediable to biological/pharmacologic and/or psychological intervention can be targeted in directed trials.