The waterborne creature, Schistosoma mansoni, hitches a ride in aquatic snail tissue before emerging into water, where it can bore into swimming or wading people, according to the U.S. Centers for Disease Control and Prevention. Inside the human body the worms can lay eggs, which escape the body via urine or feces. If the eggs land in water, the hatched worms may find their ways to a snail host, beginning the cycle anew.

The resulting disease, called schistosomiasis, is not generally lethal. But it can cause anemia, damage internal organs, impair growth and cognitive development in children, and lead to a general chronic weakening of its human hosts. (See a human-body interactive.)

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