“Mental Immunity is the perfect vaccine for the mind-viruses infecting our culture: alternative facts, fake news, and conspiracy thinking, to name a few.” —Michael Shermer, publisher of Skeptic magazine and author of The Believing Brain Astonishingly irrational ideas are spreading. Covid denial persists in the face of overwhelming evidence. Anti-vaxxers compromise public health. Conspiracy thinking hijacks minds and incites mob violence. Toxic partisanship is cleaving nations, and climate denial has pushed our planet to the brink. Meanwhile, American Nazis march openly in the streets, and Flat Earth theory is back. What the heck is going on? And what can we do about it? 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. A practical guide to spotting and removing bad ideas, Mental Immunity is a stirring call to transcend our petty tribalisms, and a serious bid to bring humanity to its senses.