Animals exhibit multiple behavioural strategies that serve as a defence against pathogens (Table II). Some of these strategies involve reflex responses, for example, to objects that may harbour pathogens, or to sick individuals. Other strategies involve learning from previous experiences, as well as observing the experiences of others. Thus these reflex responses and learned avoidance behaviours are an important component of behavioural defence, allowing an organism to respond to its environment in a way that minimizes the threat from pathogens.

The immune system may also be involved in behavioural defence against pathogens. The behavioural pattern of sick individuals, which has come to be known as sickness behaviour (Table I), clearly has defensive value and is no longer considered maladaptive. The existing evidence supports the idea that sickness behaviour is induced by the immune system, probably through the actions of certain cytokines, hormone-like factors secreted by cells of the immune system, but other factors may also be involved. Thus the immune system affects behaviour. This may explain why quite similar behavioural patterns are observed in sickness, despite very diverse causes. Sickness behaviour has much in common with behavioural patterns observed in individuals under stress, and in clinical depression. Thus sickness behaviour may be a special case of stress-induced behaviours.

The immune system-induced behavioural changes provide an important component of behavioural defence. The general depression of activity serves to conserve energy and to encourage animals to hide in safety from predators, to avoid additional injuries and infections. The increased sleep time adds to this. The lack of activity also conserves energy at a time when the animal may not be able to gather food. It may also fuel a fever, which in some cases impairs proliferation of pathogens, and may enhance the efficacy of the immune system. The general reduction in activity is bolstered by the relative lack of interest in eating, sexual activities, and exploring novel objects or environments. Thus sickness affects motivation. Nevertheless, it is clear that specific sickness behaviours may be absent if they are inappropriate for the particular circumstances of the organism.

There is limited evidence that behaviour can affect immune function. There may be some conditioning effects on immune function. Also, a general increase in the circulating concentrations of certain cytokines (e.g., IL-6) may prime the immune for action, when the environment threatens or a high-risk behaviour occurs. The net result is that, in the event that animals become infected with pathogens or injured, their behaviour changes in a manner that facilitates their recovery.

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