Giardiasis

Giardiasis is an infection of the intestine, common all over the world, caused by Giardia duodenalis. Giardia is a parasitic protozoan, meaning a single-celled organism that lives at another's expense.

It reaches a person as a cyst, a hardy form that gets swallowed. The cyst is like a hard-shelled seed that opens inside the intestine. Stomach acid triggers excystation, which is exactly that opening, and trophozoites, the form of the parasite that multiplies, emerge in the small intestine.

The trophozoite attaches to the wall of the first part of the small intestine with a large adhesive disk. That wall is covered with villi, tiny projections like the loops on a terry-cloth towel. The parasite's attachment can make them atrophy, that is, flatten those loops. As a secondary effect, there is also a deficiency of brush border enzymes, the ones that work on the surface of the intestinal cells, and that deficiency contributes to malabsorption: food passes through before the intestine can make use of it.