In Mental Immunity, Andy Norman shows that these phenomena share a root cause. We live in a time when the so-called “right to your opinion” is thought to trump our responsibilities. The resulting ethos effectively compromises mental immune systems, allowing “mind parasites” to overrun them. Conspiracy theories, evidence-defying ideologies, garden-variety bad ideas: these are all species of mind parasite, and each of them employs clever strategies to circumvent mental immune systems. In fact, some of them compromise cultural immune systems – the things societies do to prevent bad ideas from spreading. Norman shows why all of this is more than mere analogy: minds and cultures really do have immune systems, and they really can break down. Fortunately, they can also be built up: strengthened against ideological corruption. He calls for a rigorous science of mental immune health – what he calls “cognitive immunology” – and explains how it could revolutionize our capacity for critical thinking.