In Animals: hookworm larvae mature to adults in the intestine. Soon after feeding on blood the adults begin laying eggs which pass with the feces. Under favorable conditions of moisture and warmth the eggs hatch within 12 to 18 hours and a tiny juvenile emerges. This is the rhabditoform larva. It is encased within a sheath that offers protection against desiccation and ensures long life in the environment. These larvae reside in moist areas of the soil and overlying vegetation where they feed. After a few weeks the rhabditoform larva transforms into the infective filariform larva that can live for many weeks without food. These infective, non-feeding larvae are able to penetrate the intact skin of dogs or cats who step or lay upon them. The body heat of the animals excites the larva and they go penetrate through the thin skin of the ventral abdomen. The larvae then follow a blood vessel to the lungs. From there the worm is coughed up and swallowed to mature in the intestine; begin sucking blood and laying eggs.
Picture from PetStuff Online Newsletter, Vol. I Issue 3, August 27, 1999.
In Humans: humans are merely accidental hosts of animal hookworms. Infection takes place after contact with soil or sand contaminated with feces from infected vertebrates. Larvae hatch from the eggs and penetrate the epidermis and the upper dermis by producing an enzyme, hyaluronidasebut they can penetrate no further. They cannot migrate as in the normal hookworm cycle because they do not have the proteases to penetrate deeper into the human body, and so usually die after a few weeks or months.
Life Cycle taken from Diagnostic Medical Parasitology, p. 177.