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? Why is all this happening, and why now? More important, 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. Hailed as "a feast for thought," Mental Immunity melds cutting-edge work in science and philosophy into an "astonishingly enlightening and productive" solution to the signature problem of our age. A practical guide to spotting and removing bad ideas, a stirring call to transcend our petty tribalisms, and a serious bid to bring humanity to its senses.