Chapter 2 introduces readers to some one of the key threats to human security – infectious diseases. The chapter begins by defining diseases and pathogens. This is followed by an explanation of what pandemics/epidemics are and how they may differ from endemic diseases. The chapter explains the extent to which these diseases threaten human security and the major trends in infectious diseases. The chapter then offers an overview of the big three diseases (malaria, tuberculosis, and HIV/AIDS), along with neglected tropical diseases and other viral infectious diseases such as Covid-19, SARS, and Ebola. In this overview, the chapter explains how these diseases are transmitted and the symptoms they carry.