Why do people reject science and believe online conspiracy theories? Philosopher Andy Norman sets out to answer these questions. Professor Norman has spent years studying bad ideas and how we come to them. His paradigm-shifting diagnosis for society is that we are infected by mind-parasites, ideas that can poison our minds and leave us more susceptible to conspiracy thinking, make us reject scientific evidence, and to hold on to our unfounded beliefs. He looks at the philosophy of productive discussion, psychological evidence about how to change minds, and discusses how we can safeguard our minds from infection, as well as help to protect those around us from mind-parasites like conspiracies and fake news. Norman offers suggestions to create a value- and respect-based system of dialog that can help us bolster cultural cognitive immunity as we protect our own mind from bad ideas that might infect them.