Over the past 20 years, psychoneuroimmunological research has produced a large body of evidence that challenges the historically dominant view that the immune system operates in an autonomous manner independent of other physiological systems. Today, there is little doubt that the brain and the immune system are intimately linked and capable of reciprocal communication (Ader et al. 1991). Despite the acknowledged bi-directional nature of the brain–immune system connection, the predominant focus of study has been on the effects of psychological and behavioural events (e.g. stress) on immune responses and disease processes, and the mechanisms underlying such effects (see Kusnekov & Rabin, 1994; Maier et al. 1994; Rozlog et al. 1999). However, considerable interest in the possibilities of immune-system-to-brain communication was initiated by a seminal paper considering the biological basis of behaviour in sick animals (Hart, 1988). Subsequently, the immunological determinants of the behavioural, cognitive and emotional changes associated with acute illness, as well as with more chronic psychopathological states (e.g. depression) have become the subject of rapidly expanding areas of research (e.g. Kent et al. 1992; Lloyd et al. 1992; Hickie & Lloyd, 1995; Maes et al. 1995a; Rothwell & Hopkins, 1995; Dantzer et al. 1996; Maier & Watkins, 1998; Vollmer-Conna et al. 1998; Maes, 1999).

The main objective of this editorial is to provide a succinct overview of current knowledge of the normal behavioural correlates of acute infective illness, their adaptive function and underlying mechanisms. Elucidation of the processes involved in the appearance, maintenance and inhibition of ‘normal’ sickness behaviour is important if extrapolations from this phenomenon to more chronic psychopathological conditions are to provide more than a new label for poorly understood non-specific symptom clusters.