Technically speaking, behavior altering parasites can be categorized as any parasite that causes the host to perform actions that it does not usually perform. By this definition, most parasites are behavior altering: fleas make your cat scratch themself, strep throat makes you stay home in bed. A narrower definition is often applied, however, as any parasite (usually an endoparasite – one that lives inside the host) that changes the behavior of the host using a variety of physiological and chemical properties, and does so in a way that benefits the parasite. A few of these parasites are well-known in pop culture as “zombie parasites” – Cordyceps (usually Cordyceps sinensus or Ophiocordyceps unilateralis) and snail-infecting flatworms (Leuchochloridium paradoxum) among them. But how do they do it? Why do so many of them cause the same behaviors? How did they ever evolve to be this weird and scary? I’m going to overview the neurophysiological mechanisms behind some of nature’s biggest brain-changers, from single celled protozoa to spider-jacking wasps.