There is no small number of unsettling parasites crawling, flying, and swimming about the Earth, the lucky ones hitching rides on hapless host organisms. And while many parasites are harmless, or even beneficial in their way, others can wreak havoc with their hosts' existence. The worst of these offenders can actually force their host to do their will.
A tiny barnacle called Sacculina is one such parasite. Upon finding a host crab, a female Sacculina will crawl over the crab's surface until she finds a chink in the armor: a joint. She then ejects her protective shell, reducing herself to a gelatinous blob, and invades.
Inside the host, the parasite grows long, root-like tendrils throughout the crab's body, eventually emerging as a bump on the its underside. During this process she renders the crab infertile, and creates a small opening in the crab's back that will allow a male Sacculina to make residence there. Soon the crab is filled with millions of Sacculina eggs and larvae, and like a zombie, the crab cares for these eggs and larvae as though they were its own, losing all interest in mating. When a male crab is infected, the parasite alters its physiology and behavior to be female, to better care for the Sacculina's young.
The parasite basically rewires the crab for its own ends, and the crab becomes a helpless vehicle, expending its energy caring for the young organisms that will move on to inflict themselves upon other crabs.