Human beings are a brilliant species, but also a dangerously unfinished one. Again and again, societies place extraordinary power in the hands of people who deceive, exploit, divide, and destroy. We tell ourselves that the problem is a few bad leaders. But the deeper question is more troubling: Why do we keep following them? The Social Immune System argues that every society depends upon a kind of collective immune system: the capacities, institutions, relationships, and moral habits that help people recognize danger, distinguish threat from difference, contain predatory power, preserve memory, and repair damage before harm spreads. When that system is healthy, a society can protect itself without becoming cruel. When it is weak, captured, or distorted, predators flourish. Drawing on developmental science, trauma theory, history, politics, and the biology of the human immune system, psychiatrist and author Sandra L. Bloom offers a new framework for understanding authoritarianism, institutional failure, social amnesia, and democratic vulnerability. But this is not only a diagnosis. If democracy is a developmental achievement, then societies can become more discerning, more resilient, and harder for predators to govern. The Social Immune System asks what it would mean for human beings not merely to survive power, but to grow up enough to use it wisely.