‘Contagion’ is a crucial term in the theory of networks, that is nowadays employed to study increasingly diverse issues such as financial crises, epidemics, and fake news. It provides a unifying framework to describe various social and economic phenomena that occur within different networks — involving various forms of contagion. This book discusses contagion over social and economic networks, and its consequences for modern societies. It starts with an accessible but solid introduction to basic concepts of the theory of networks, with intuitive explanations and examples both of networks that shape our daily lives, and of how researchers analyze them. It then focuses specifically on contagion, and on its importance for societies. Three crucial examples are analyzed more in depth: epidemiological contagion, fake news and financial crises. The conclusions show how various societal problems — each consequence of some form of contagion — call for similar policy measures: a pre-emptive and comprehensive effort in collecting the required information, and a shift in the concept of individual responsibility itself. This is a book for any reader who is interested in how contagion and networks shape modern society.