There are a lot of parasites that cause strange changes in their hosts. Parasites turn hosts into zombiesgorge on the flesh of the host from the inside out, even assist a host's suicide. But one of the most interesting and extreme changes caused by a parasite is achieved by the mermithid nematode. Simply put: it makes dude mayflies look like ladies.
You see, like many other nematodes, the adults of the species are free living. But their young enjoy all the comforts of living off the hard work of other species, specifically the mayfly. Mayflies are known for their peculiar life cycle - they spend about a year as a wistful youth, swimming around freshwater systems, before spending a brief and sex-filled day or two as an adult. After fleeting joys, the female flies deposit their eggs back into the water, while the males simply die wherever they are.
This poses a slight problem for a parasite that lives in water most of its life. You see, mayfly nymphs eat the eggs of the nematode accidentally in their long and drawn out youth, and carry the worm until their deaths. When the nematode infects a female fly, it waits around until she goes to put her eggs in the water, growing all the while, then busts free through her abdomen in a very alien-esque manner. But male flies don't deposit eggs. Since infection occurs by the fly nymph ingesting the parasite's eggs, there's no way for the parasite to control ending up in only female flies. If the parasite infects a guy, it'd be SOL if it had no way to get the boy to water.
I say it would be, because, of course, the parasite has a way of guaranteeing it gets what it wants. It does this by turning the male flies into females. Infected males have feminized features, mate with other male flies, and then fly dutifully to deposit their "eggs" in the nearest stream. Somehow, the worm disrupts the normal male hormonal signals and turns the guys into gals to get what it needs. How, exactly, the parasite does this remains a bit of a mystery.
Just imagine if there was a parasite that could turn human men into women, not just visually but behaviorally, too, all for its eventual goal of using our bodies as food and safe keeping for its growing young. *shiver*