It is a term introduced by the psychological scientist Mark Schaller, describing mechanisms devised by animals, including humans, to counter microbes that cause infection. A simple example is the repulsion towards rotten food.
The behaviour immune system may be considered complementary to the body’s immunological defence. The latter consumes energy and is reactive; the pathogens first enter, and then the body produces compounds (e.g. antibodies) to counter. But a repulsive smell or taste prevents some from consuming it in the first place.
References
Mark Schaller, Phil. Trans. R. Soc. B (2011) 366, 3418–3426
Behavioural immune system: Wiki
