Boundary lines between medicine and social sciences have often been crossed in either direction during the course of their history. Lately the re-discovery of social determinants in health inequalities recalls the tradition of the socio-medical inquiries that that characterized the XIX and the XX century, while the epidemiological scenario of the latter has long included social daily life functioning in the evaluation, prevention and care of most important chronic diseases. Vulnerability is a new concept for the explanation of health inequalities. It is not a causal but a dispositional concept so it triggers events under certain environmental circumstances; it is systemic in its essence, being complementary to resilience; it was first applied to the socio-ecological environments of the third world. It was recently regarded as suitable for the western post-modern risk societies in health sociology in particular by describing both systems undergoing fatal shocks (i.e. pandemic/epidemic risk fatalities) as well as vulnerable trajectories throughout individual course of lives. By including basic factors such as social determinants of inequality vulnerability contends with frailty its importance in medical sciences, while, as a disposition, it is increased or decreased by socio-structural as well as individual action. Its inclusion in health sociology studies requires middle range theories such as those on coping and on social capital both at the individual and community level.
Keywords: Vulnerability, fragility, life trajectories, social determinants, health inequalities, medical sociology.