Sickness and depressionOver the past 30years, it has become clear that the immune system plays a critical role in animal behavior. This role is tightly linked to the obvious role that immune cell activation plays in the clearance of pathogenic organisms. Systemic or central infections elicit a group of symptoms that are necessary for the organism to conserve resources, reorganize priorities and limit the spread of the infection to other members of the community. This sickness behavior is a motivational state that is common to most pathogeninduced infections ranging from viruses to multicellular parasites, but, because of its ubiquitous nature, is frequently accepted as an unavoidable and non-specific consequence of infection. However, considering the broad spectrum of symptoms -fever, nausea, decreased appetite, malaise, fatigue and achiness -it seems clear that a highly organized, although not pathogen-specific, response is being manifested to aid in the fight against infection (Dantzer, 2001;Ericsson et al., 1995).We are all familiar with the human symptoms of sickness but, to investigate changes in behavior associated with sickness, it is critical to have reliable animal measurements that relate to changes in the affective state. Using preclinical animal models, sickness behavior is best evaluated when the test involves a means to assess motivation. Sickness behavior is frequently assessed as social exploration/investigation (in rodent models, this response is frequently reported as a decrease in time actively seeking interaction with a novel animal as a result of diminished motivation for social exploration or neophobia). Social exploration is a naturalistic behavior and all animals have a strong motivation, whether driven by curiosity or sexual desire. With this strong motivational stimulus, infections cause a strong differential between time of exploration of healthy and sick animals, with less time of exploration being observed with sick animals. Another simpler but effective model is exploration of the environment [often referred to as locomotor activity (LMA), representing some level of physical movement in a novel environment such as rearing, quadrant entry, line crossings or total distance traveled; all of which are highly correlated]. LMA is best conducted in a novel environment, as the motivation to explore is stronger in this