The behavioral immune system is a phrase coined by psychologist Mark Schaller of the University of British Columbia to refer to a suite of psychological mechanisms that allow individual organisms to detect the potential presence of disease-causing parasites in their immediate environment, and to engage in behaviors that prevent contact with those objects and individuals (or remediate their effects).
These mechanisms include sensory processes through which cues connoting the presence of parasitic infections are perceived (e.g., the smell of a foul odor, the sight of pox or pustules), as well as stimulus–response systems through which these sensory cues trigger a cascade of aversive affective, cognitive, and behavioral reactions (e.g., arousal of disgust, automatic activation of cognitions that connote the threat of disease, behavioral avoidance).
Such systems are common to a wide range of animals, and it is theorized they evolved as a crude first line of defense against disease-causing pathogens. In humans, there is extensive research linking the behavioral immune system to a variety of prejudices—including prejudices against people who aren’t actually diseased but are simply characterized by some sort of visual characteristics that deviate from those of a subjectively prototypical human being. The disease–avoidant response has been associated with discrimination against the obese, elderly, and disabled or disfigured. In addition, the behavioral immune system appears to contribute to xenophobia and ethnocentrism.
These prejudices tend to be exaggerated under conditions in which people feel especially vulnerable to the potential transmission of infectious diseases (such as in a dirty, dank room). For instance, when the potential threat of illness is made salient, people tend to be less extraverted or sociable. The behavioral immune system also has consequences at a cultural level of analysis. Under ecological circumstances in which diseases are more prevalent, societies in general also tend to display more reticent and socially restricted forms of behavior, and are defined by more conservative norms and value systems.
Some research suggests that the behavioral immune system has implications for the functioning of the physiological immune system too. One study found that the mere visual perception of diseased-looking people stimulated white blood cells to respond more aggressively to infection. Research also indicates that immune-relevant interventions which target pathogen transmission can interrupt behavioral responses. For example, receiving a flu vaccination or washing one’s hands can reduce the extent of negative interpersonal and intergroup attitudes elicited by disease cues and concerns.
The Daily Omnivore 
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