'Not all internal parasites kill their hosts, of course: pretty much every multicellular animal is home to numerous fellow travelers, each of which has its own agenda, which in some cases involves influencing, or taking control of, part or all of the body in which they temporarily reside.

And this, in turn, leads to the question: who’s in charge of your own mind? Think of the morgue scene in the movie “Men in Black,” when a human corpse is revealed to be a robot, its skull inhabited by a little green man from outer space. Science fiction, but less bizarre than you might expect, or want to believe.'

Read more: Who’s in Charge Inside Your Head?