A blood fluke starts out as an egg in human feces. If it happens to fall into fresh water it will live only if it happens to encounter a certain species of snail. It changes in the snail, swims out, and now needs to find a human being in the water in order to bore through his skin. It travels around in the man’s blood, settles down in the blood vessels of his intestine, and turns into a sexually mature blood fluke, either male or female. Now it has to find another fluke, of the opposite sex, who also just happens to have traveled the same circuitous route and landed in the same unfortunate
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