Hookworm is one of most successful human parasites, having been around for many thousands of years and today residing in the intestines of close to a billion human beings. The two main species are Ancylostoma duodenale and Necator americanum.

Hookworm would not be so prevalent were it not for several persistent habits of human beings. The first is the habit of defecating outside on the ground, and the second is the practice of using untreated human sewage as fertilizer for crops. These two things, so ingrained in the cultural habits of many societies around the world, account for the majority of hookworm infection worldwide.

Deposition in the soil plays right into the scheme of the hookworm lifecycle. Adult hookworms are seldom seen because they are quite tiny (a female is only about 1 cm long, and the males are even smaller); they remain in the intestine clinging to the lining with their wide mouths and grasping teeth. Females produce many eggs, which are passed out with the feces onto the ground. There the parasite will infect its next host if conditions are right.

In warm moist conditions, hookworm larvae emerge from the eggs and develop quickly to infective larvae. They wait at the surface of the soil as the feces gradually break down, waving their bodies in the air in anticipation of the opportunity to infect a new host if one wanders by