Pinworm

The human gastrointestinal tract (primarily the colon and rectum) and household environments (bedding, clothing, dust) after excretion.

It is a parasite that feeds on the intestinal contents and epithelial cells of the host's large intestine.

Adult females migrate out of the anus at night to lay thousands of eggs on the perianal skin. This causes itching (pruritus ani), leading to scratching and unintentional ingestion or inhalation of eggs, completing the life cycle.

Risks include intense itching, sleeplessness, and secondary bacterial infections from scratching. In rare cases, they can migrate to other organs. They offer no ecological benefits and are considered a common human health nuisance.