Hookworm disease exists in tropical and subtropical areas of the world where there is sandy soil that contains lots of organic material, and rainfall adequate to keep the soil from drying out too much. Infective larvae are very susceptible to drying and to extremes of temperature; therefore, a person who has not lived in or traveled to such a region is very unlikely to have hookworms. It is important to remember, however, that hookworm infection can be transmitted locally in other areas, during the warmer seasons of the year, if conditions are right.

A light hookworm infection (a couple of dozen worms in the intestine) will go unnoticed and is not likely to compromise health. To describe symptoms of hookworm infection for heavier infections, it's necessary to divide hookworm disease into two separate time periods: the period immediately after infection when the larvae are migrating and maturing to adult worms, and the period of chronic infection, when the adult worms are established in the intestine.

Hookworm larvae penetrate human skin when people walk bare-skinned on moist soil or through damp vegetation where the larvae are waiting for just such an event. An itchy rash appears at the site of penetration, which may become infected with other organisms if the individual scratches. The larvae, meanwhile, have moved on, traveling to the lungs in the bloodstream and breaking out into the airspaces of the lungs.

The presence of many larvae in the lungs can cause inflammation that can look like a severe pneumonia in extreme cases, but it is usually mild. The most notable symptom at this stage is a slight to severe cough. Pneumonitis clears up as the larvae pass up the bronchi and are swallowed.

The final stage of the worm's lifecycle, and of hookworm disease, begins as the larvae pass into the small intestine. Each larva attaches itself to the intestinal lining by taking a mouthful of tissue and hanging on. The tissue breaks down and blood begins to flow and feed the young worm that will soon be mature. At this stage, the victim may suffer fatigue, abdominal pain, diarrhea, vomiting, and bloody stools. Small children and individuals in an already weakened state of health are in grave danger at this point, especially if the number of worms is very high.

Finally, chronic hookworm disease sets in, characterized by listlessness and anemia, pale skin tone, and swollen face and feet. The adult worms continue to draw blood, and blood loss continues from their sites of attachment in the intestine. Female worms produce many eggs, which are passed in the stool.

During the early stages of infection, diagnosis of hookworm disease depends on clinical symptoms along with appropriate travel or exposure history. In the absence of symptoms, an individual who has traveled in an endemic area may desire screening for the infection, but it will have to wait. The definitive test is examination of stool for hookworm eggs, and it takes about five months for worms to mature and begin producing eggs. In some cases it may take even longer.

Little know facts about hookworm infection include the fact that at least one species can remain dormant in the muscle tissue of common domestic animals such as beef cattle and pork: if these animals are slaughtered and the meat consumed without being thoroughly cooked, the larvae will infect the new host. It also appears that the same species is capable of passing from mother to fetus during pregnancy - some people may be born with hookworm infection.