Many parasites depend on their host’s behavior in order to successfully reproduce. Instead of leaving this behavior to chance, some parasites actively manipulate their hosts to produce the desired behavior. For example, after infecting a rat, the taxoplasmosa gondii parasite needs to be transferred to a cat’s belly to reproduce. To do this, the sneaky parasite rewires its rat host to actively seek the smell of cat urine. When the rat gets eaten, the parasite completes its necessary transfer.

Cordyceps fungi infect insects and steer them to higher ground where, when the insect dies and the fungus bursts forth, the fungus spores will be more effectively dispersed by wind.

Another parasite—this one a mite of wasps—is transmitted sexually, after which it acts as a sperm plug to prevent further fertilization (and thus competition).  

The candiru fish has seen pop-culture spotlight for obvious reasons—Grey’s Anatomy referred to it as the “penis fish.” The eel-like fish follows a water source to its home in a host’s orifice (you do the math), where it becomes impossible to remove without surgery due to its barb-like spines.

The fish parasite Cymothoa exigua attaches to the tongue of the rose snapper and steals the tongue’s blood supply. When the snapper’s tongue eventually atrophies and falls off, the parasite attaches itself in the tongue’s place, effectively becoming the fish’s tongue, where it shares meals with its host.

When it comes time for the wasp Ampulex compressa to lay her eggs, she finds a cockroach to act as host. But instead of simply laying her eggs in the unfortunate roach, she fist lands on his back and inserts a specialized stinger into the roach’s brain, which she then uses to steer the roach—under its own power—back to her burrow, where the zombie roach sits placidly as the wasp larvae gobble his living innards.

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