This chapter presents the concept of the fourth world as a useful framework for identifying and addressing health disparities that affect the most marginalised populations across societies. It discusses the rise in chronic hepatitis C virus (HCV ) infection among people who inject drugs in settings such as the United States, Canada, Australia, and the United Kingdom as a product of political, economic, social, and cultural marginalisation. The chapter highlights the importance of social mobilisation to generate political will to address HCV treatment access. It focuses on the fourth world inhabited by people who inject drugs living with HCV in so-called developed countries. The striking health disparities that require global health solutions exist within pockets of deep social and economic exclusion that exist in every society – what Manuel Castells termed the ‘fourth world’. The fourth world framework proposed by Castells is useful for understanding the politics of global health on several levels.