The aetiology of FSS is assumed complex, with interacting biological, psychological and environmental factors. Recent studies suggest that dysfunction of the stress-axes (e.g. the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system) and activated inflammatory response are likely to play a role in the development and perpetuation of the symptoms in various FSS. Besides common pathophysiological mechanisms, FSS also show similarities in patient characteristics and treatment response, which speaks in favour of a common family of disorders. Recently, the unifying diagnostic category Bodily Distress Syndrome (BDS) was introduced. BDS is conceptualized as a (patho)physiologic response to prolonged or severe mental and/or physical stress in genetically susceptible individuals, and the diagnosis has been shown to encompass the majority of FSS.