Infectious disease is a moving target: new diseases emerge every year, old diseases evolve into new forms, and ecological and socioeconomic upheavals change the transmission pathways that spread disease. But where does disease come from? How is it transmitted from one person to another? And why are some individuals more susceptible than others? In this Very Short Introduction, Marta Wayne and Benjamin Bolker address these questions through the lenses of ecology and evolution. Assessing the management of outbreaks of diseases such as influenza, HIV/AIDS, cholera, and COVID-19, they provide specific examples to illustrate why major diseases still threaten populations all over the world.