6. Ascariasis

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Like many parasites, this one may enter the body as a tiny organism that goes unnoticed, but it ultimately develops into a worm that looks a bit like the common earthworm and can grow to up to a foot long! This is the most common parasitic worms in humans and it often causes difficulty breathing and fever when the larvae attack the lungs. When they migrate to the intestines later in their miserable parasitic lives, they grow into the foot-long monsters that you really don’t want to see in your toilet some day! It is estimated that up to a billion and a half people are infected in the world.