Hookworm infection begins when the worm is in the larval stage. It penetrates the skin and migrates during its life cycle through the liver and the lungs, and it attaches to the mucosa of the small intestine where it matures. Hookworms deplete the body of nutrients, and a major effect is severe chronic iron-deficiency anemia.
Necator americanus, which ranges in size from 5 to 11 millimetres (0.2 to 0.4 inch), is responsible for about 90 percent of human hookworm infections that occur in tropical and subtropical regions of the world. Ancylostoma duodenale, 8 to 13 millimeters long, is found on all continents but is most prevalent in warm regions. A. braziliense, from 8 to 11 millimetres long, is normally parasitic in dogs and cats; man, however, is sometimes infected by this species in the southern United States, South America, and Asia. A. ceylanicum, normally parasitic in dogs, is sometimes found in man in South America and Asia. A. duodenale, possesses four hooklike teeth in its adult stage, and N. americanus has plates in its mouth rather than teeth.
Both species of hookworm have similar life cycles. The adult worm attaches itself to the mucosal tissue lining the small intestine, where the female may produce several thousands eggs a day, which are passed in the feces. If the fecal matter reaches suitable soil, the eggs are hatched, and the infective, threadlike larvae may penetrate human skin, usually that of the foot, by way of the sweat glands and hair follicles. They then invade the lymph and blood vessels, reach the lungs, and pass up the respiratory tree to reach the mouth, where they are swallowed and sent to the small intestine; there they mature and start a new reproductive cycle. The intestinal organisms are reputedly long-lived, with a lifetime that may span some 10 years.
The symptoms of hookworm disease ordinarily begin with ground itch, an itchy skin irritation caused by the larvae when they penetrate the skin and marked by papules and vesicles that are often located between the toes. In passing through the lungs, the larvae may produce coughing and fever. In the intestine, the mature worm sustains its life by bloodsucking, and persistent feeding by many worms over many years results in secondary anemia.
In heavy infestations (which may involve more than 500 larvae), the general symptoms include pallor of the skin and mucous membranes, fluid retention in the face and extremities, constipation alternating with diarrhea, abdominal tenderness, increased appetite for bulky or strange substances (e.g., clay), delayed puberty and stunted growth, fatigue, dullness, and apathy. Hookworm infestation tends to be constantly widespread in varied regions of the world.
Life Cycle Diagram (Courtesy of the DPD)
Eggs are passed in the stool 
, and under favorable conditions (moisture, warmth, shade),
larvae hatch in 1 to 2 days. The released rhabditiform larvae grow in the feces and/or the soil 
, and after 5 to 10 days (and two molts)
they become become filariform (third-stage) larvae that are infective 
These infective larvae can survive 3 to 4 weeks in favorable environmental conditions. On contact with the human host, the larvae penetrate the skin and are carried
through the veins to the heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree to the pharynx, and are swallowed
The larvae reach the small intestine, where they reside and mature into adults. Adult worms live in the lumen of the small intestine,
where they attach to the intestinal wall with resultant blood loss by the host 
.
Most adult worms are eliminated in 1 to 2 years, but longevity records can reach several years.
Some A. duodenale larvae, following penetration of the host skin, can
become dormant (in the intestine or muscle). In addition, infection by
A. duodenale may probably also occur by the oral and transmammary
route. N. americanus,
however, requires a transpulmonary migration phase.
