This Is Your Brain On Parasites: How Tiny Creatures Manipulate Our Behavior and Shape Society

by Kathleen McAuliffe

With firepower like that you’d think humans would always be on the winning side. But parasites have huge advantages over us. Their population size dwarfs our own by staggering numbers, and their rapid replication rates ensure that there will always be a lucky few with mutations that will give them the upper hand. The battle between hosts and parasites is an unending arms race. In this intensely competitive environment, any parasites that by chance hit on ways to modify the behavior of a host so as to enhance their own transmission—perhaps, for example, by nudging it a wee bit closer to the parasites’ next host—would very swiftly multiply. Since hosts can’t evolve as quickly to thwart every new trick parasites deploy against them, their best chance for survival is to acquire traits that offer them broader protection. Mutations that prompt an animal to feel repelled by common sources of contagion—for instance, murky green water, a dung heap, or other members of its flock acting strangely—might serve that function.