Those worms, at least some of them, also change it's host's behaviour so that it seeks water and drowns itself when the parasite is ready to face the world. Other parasite lives some time in cow's intestines, then burrows itself into it's liver and nests there over a winter. In spring it lays it's eggs in liver and blood carries those out with shit. Then those insects that eat it will be infected and their behaviour changes so that they go on the top of grass or flowers. And why? So that cows eat them and the whole cyclt starts again. This all damages cow's liver and it dies sooner than usual. What a wacky world.
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