Pinworm

The human large intestine; they are frequently found in and around the anal area of humans, as well as on contaminated surfaces like bedding and clothing.

Intestinal contents, bacteria, and epithelial cells within the human host.

The female crawls out of the anus at night to deposit thousands of eggs on the surrounding skin. This causes itching (pruritus ani), which leads the host to scratch and inadvertently transfer eggs to hands and surfaces, facilitating the life cycle's continuation.

Risk: Causes Enterobiasis, which includes symptoms like itching, restless sleep, and irritability. It is highly contagious among household members. Benefit: None to the host; they are obligate parasites.