This chapter examines the impact of health on security. It first considers how health and human security are connected via diseases such as HIV/AIDS, malaria, and tuberculosis. In particular, it looks at health security, economic security, and food security. It then describes some recently emerging infectious diseases, such as SARS, influenza pandemic, and Ebola, that are now also recognized as threats to national security. It also discusses diseases that are known to pose narrower threats to bio-security within the context of international efforts to combat terrorism, focusing on disease-causing biological agents such as anthrax, smallpox, and plague. The chapter concludes by contrasting two different ways in which the health-security nexus can be understood. Two case studies are presented, one relating to the impact of HIV/AIDS on the South African National Defence Force, and the other relating to the Aum Shinrikyo cult’s Sarin nerve gas attacks in Tokyo.