Ninety years after the discovery of human influenza virus, Modern Flu traces the history of this breakthrough and its implications for understanding and controlling influenza ever since. Examining how influenza came to be defined as a viral disease in the first half of the twentieth century, it argues that influenza’s viral identity did not suddenly appear with the discovery of the first human influenza virus in 1933. Instead, it was rooted in the development of medical virus research and virological ways of knowing that grew out of a half-century of changes and innovations in medical science that were shaped through two influenza pandemics, two world wars, and by state-sponsored programs to scientifically modernise British medicine. A series of transformations, in which virological ideas and practices were aligned with and incorporated into medicine and public health, underpinned the viralisation of influenza in the 1930s and 1940s. Collaboration, conflict and exchange between researchers, medical professionals and governmental bodies lay at the heart of this process. This book is a history of how virus researchers, clinicians, and epidemiologists, medical scientific and public health bodies, and institutions, and philanthropies in Britain, the USA and beyond, forged a new medical consensus on the identity and nature of influenza. Shedding new light on the modern history of influenza, this book is a timely account of how ways of knowing and controlling this intractable epidemic disease became viral.