These days, whenever anything spreads, whether it's a YouTube fad or a political rumor, we say it went viral. But how does virality actually work? Here epidemiologist Adam Kucharski explores topics including gun violence, online manipulation, and, of course, outbreaks of disease to show how much we get wrong about contagion, and how astonishing the real science is. Why do financial bubbles take off so quickly? Why are disinformation campaigns so effective? And what makes the emergence of new illnesses—MERS, SARS, or COVID-19—so challenging? By uncovering the crucial factors driving outbreaks, we can see how things really spread, and Kucharski explains what we can do about it.