Through mechanisms that remain mysterious, then, the impatient parasite induces a precise sort of madness in its host. It prompts the poor creature to defy all known pill bug precautions, to venture forth in broad daylight and crawl onto a light-colored surface, where the dark pill bug stands out so starkly it might as well flash a sign saying, Snack Time! As a result, infected pill bugs end up being eaten by birds at a far higher rate than are their unparasitized peers, and the thorn-headed worm lives on.

A similar story applies to the horsehair worm, which thrives as a larval youth inside a cricket or grasshopper, but must be delivered to a stream or lake to breed. Healthy crickets and grasshoppers are devoutly terrestrial and do not go swimming. A parasitized insect, however, finds itself with the odd compulsion to head for the nearest body of water and jump in, at which point the ripened, writhing worm bursts its host apart like a comic book villain rupturing from a mild-mannered disguise, and starts seeking another freshly born-again worm.

The tiny brains of invertebrates may seem easier takeover targets than the complex counterpart organs in mammals, but parasites have both opportunity and motive to succeed. Their generation times are quite short compared with a mammal’s, allowing them to evolve new tricks faster than mammals can combat them, and their finicky needs can often be fulfilled only by manipulating host behavior. In the case of Toxoplasma gondii, the parasite appears to have hit on the perfect method for puppeteering a common primary host — the rat.

More than most mammals, rats habitually dig around in garbage and animal waste, where traces of cat fecal matter laced with Toxoplasma eggs may reside. But how to journey reliably at maturity from rat to cat, when eons of evolution have hammered into rats a keen aversion to the sight and smell of live cats? The parasite deftly knocks that antipathy out. Studies show that while parasitized rats behave much like their healthy peers in aggressiveness and other measures, they differ from ordinary rats in one outstanding trait: they will recklessly frolic in a cat-scented room, and should a cat enter, the rattled rat is as apt to run toward its old foe as away.

We humans are not at risk of being predated on by house cats, and Toxoplasma probably has scant incentive for specifically modulating our behavior. Yet because our brains are wired much like a rat’s, infection with Toxoplasma could well exert a behavioral or temperamental effect indirectly.

As Dr. Zuk recounts, when Jaroslav Flegr of Charles University in Prague administered personality tests to two groups of people, one showing immunological signs of a prior Toxoplasma infection and the other not, infected men scored comparatively higher than uninfected men in traits like suspicion of authority and a propensity to break rules, while infected women ranked relatively higher than noninfected women in measures of warmth, self-assurance and chattiness. This last finding has prompted speculation that there may be a biological basis for the Cat Lady stereotype, the loopy, friendly neighbor who keeps dozens of cats. She loves them, they’re her children, and she’s clearly inured to their smell.