Leprosy deeply impacted on the lives of medieval European populations, even though its prevalence declined after the thirteenth century. Social attitudes and responses to the disease varied in time and space, making its history and the reason for its decline difficult to reconstruct or explain with any certainty. One hypothesis for this decline is cross immunity between the biologically related pathogens responsible for leprosy and tuberculosis; that is, cases of leprosy may have diminished due to the increasing prevalence of tuberculosis. However, without ruling out some partial impact of tuberculosis, other environmental, social and biological factors should also be considered when examining changes in the prevalence of leprosy. This chapter explores how different biosocial factors affected – either increased or decreased – an individual’s capacity to mount a proper immunological response against leprosy, in order to explain changes in its prevalence in medieval Europe. In particular, the chapter uses a syndemic approach to consider how a set of different biosocial factors could have worked together, synergistically (or counter-synergistically), to affect an individual’s capacity to fight leprosy.

This chapter does not offer a complete historical and bioarchaeological review of leprosy in medieval Europe. Instead, it uses historical and bioarchaeological evidence to explore how larger bioecological environments and social structures converged to produce infected or uninfected spaces. This, in turn, reveals that complex and heterogeneous immunological landscapes marked the disease’s prevalence. The syndemics approach helps to expand the ‘immunity’ concept by replacing it with ‘immune competence’. It also generates a new dialogue among and between disciplines such as immunology, bioarchaeology and history to reconstruct potential interactions between them. It equally helps to identify mortality burden contributions of chronic infectious diseases and biosocial factors in past populations.

Hansen’s disease is an infectious disease caused by Mycobacterium leprae, which com-monly generates a chronic infection primarily affecting nerves and skin.