The ‘Introduction’ explains what infectious diseases are: diseases that are transmitted from one person to another. For most of human history, diseases could only be controlled at the population level, using quarantines to separate uninfected from infected people. The discovery of immunization, and later the invention of disease treatments like antibiotics, allowed individual-level infectious disease control. Individual-level control can filter up to the population level: if enough of the population are vaccinated, we can reduce transmission enough to stamp out a local epidemic, or even to wipe a disease out globally. But both humans and infectious disease agents are living organisms that undergo ecological and evolutionary change, making infectious disease a moving target.