Hookworm

Hookworm is a parasitic worm that lives in the small intestine of its host, which may be a mammal such as a dog, cat, or human. In children hookworms cause intellectual, cognitive and growth retardation, intrauterine growth retardation, prematurity, and low birth weight among newborns born to infected mothers.

Three species of hookworms commonly infect humans: Ancylostoma duodenale, Necator americanus and Strongyloides stercoralis. A. duodenale predominates in the Middle East, North Africa, India and (formerly) in southern Europe, while N. americanus predominates in the Americas, Sub-Saharan Africa, Southeast Asia, China, and Indonesia. The A. braziliense and A. tubaeforme species infect cats, while A. caninum infects canines.

Hookworm in "Hi, Colonic"
Two aliens arrived on the third planet from a medium-heat sun in the search for intelligent life. While the planet had been explored before, the two were under orders to explore again. They found three in a swamp, paralyzed them, and brought them aboard the ship. After consulting their manuals, the two peeled the locals, and probed them. The first two showed no readings, but the third one did.

The explorers conversed with the intelligent lifeforms they'd found. The intelligent lifeforms assured the explorer that they were quite happy with parasitism, rather than the free-living state the explorers had evolved into. The explorers returned the three hosts to the planet, lamenting the paperwork they would have to fill out now that they'd discovered intelligent life.