Hookworm infection, the presence of hookworms in the intestine, is very common in humans, especially in tropical climates. There is a difference, however, between hookworm infection and hookworm disease: a few hookworms generally do not cause any symptoms or illness in a human host, whereas large numbers of the worms cause hookworm disease.

In order to understand the symptoms of hookworm disease, it's important to understand how hookworm infection occurs (hookworm lifecycle). You do not catch this parasite through direct contact with another infected human. Rather, you pick it up when your skin comes in contact with soil contaminated with the feces of an infected person. Hookworm larvae, the stage that infects people, are present in warm moist soil where an infected person has defecated outside on the ground.

Hookworm larvae do best in loose soil that contains plenty of organic material. They don't do well in dry sandy soils, or soils that contain a lot of clay, and they don't develop at temperatures below about 17 degrees Celsius. Freezing kills them. Because they are so sensitive to cold temperatures, hookworms are basically confined to warm climates and to warm microclimates such as soil inside mines. Hookworm infection does not occur in temperate northern and southern climates.

When human skin comes in contact with live hookworm larvae in soil, the larvae penetrate the skin and start to migrate. If there are a lot of them, there may be a skin reaction, and inflammation where they penetrate the blood vessels. The body's immune system causes this inflammation as it tries to kill the larvae. Hookworm infection may end at this stage, if the immune system is successful; larvae that survive get carried off to the heart and lungs. In the lungs, large numbers of larvae can cause the first serious sign of hookworm disease.

Hookworm larvae break out of blood vessels into the alveoli of the lungs (the sac-like spaces that fill with air when you breath in). Each larva breaking through causes a tiny bit of bleeding into the lungs. A few larvae won't even be noticed, and massive numbers breaking out at once are rare, but when it does happen, a pneumonia-like illness occurs. From the lungs, the larvae travel up the windpipe to the throat and then down through the stomach to the intestine, where they grow to adult worms. At this stage, they may cause abdominal pain and diarrhea.

Again, a couple of dozen hookworms in the intestine is a light infection that won't cause any symptoms; up to 100 worms will cause mild hookworm disease, while hundreds will seriously compromise host health, and thousands may kill. Serious symptoms of chronic (ongoing) hookworm disease include stunted growth, anemia, weakness, fatigue, protein deficiency, and learning impairment. In regions where people struggle with nutritional deficiencies already, a high incidence of hookworm disease has grave consequences for the society as a whole.