Carl Zimmer in his excellent blog, The Loom:
Remember that couple you knew, the ones who went out on a date and instantly fell in love, who had been together for years and seemed as happy together as the day they met, the ones who gave you hope that you might find your own true love, the ones who made you feel that there was joy to be found in the world? And remember how one day they suddenly called the whole thing off and pretty soon were seeing other people, leaving you confused and reeling?
I’ve been having the same experience with blood flukes.
I first encountered blood flukes while doing research for my book Parasite Rex. They are extraordinary flatworms that get their start in life in ponds and streams. Once they’ve hatched, they seek out a snail and plunge into its guts to feed. They develop and produce a new generation of flukes that look like little missiles. A single fluke can produce thousands of these missiles, which emerge from the snail and flick around in the water in search of human skin. When they find their target, they drill into their host like diving through butter. They reach a blood vessel and then ride through the circulatory system until they find their ultimate destination–depending on the species, that’s the blood vessels behind the intestines, or behind the bladder.
More here.