11. Pinworm

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The pinworm, also known appropriately as the seatworm in the United Kingdom, is another parasite that resides in the digestive tract. Humans become infected by ingesting the eggs which are invisible to the naked eye but are quite resilient, and can be spread easily by numerous means. The adult worms mate in the small intestine, which is usually followed by the death of the male. The female then migrates down through the digestive system until it emerges and lays its eggs on the anus, which often leads to itching and irritation. Oftentimes the tiny eggs are then spread around through contact with other objects, with the lucky ones find their way onto things that end up being eaten or put in someone’s mouth. Some eggs actually reverse direction and make their way back up into the digestive tract after hatching where the cycle begins again.