Giardiasis is an infection of the small intestine by the protozoan Giardia lamblia. Predominant symptoms are diarrhea and flatulence, but about two thirds of infected individuals develop no symptoms. In some cases the infection becomes chronic. Often no treatment is necessary, but antimicrobials (metronidazole and quinacrine) are sometimes prescribed. The organism is spread via the fecal-oral route. Considered a tropical disease, giardiasis is becoming more common in developed countries.

Giardiasis (also called giardia) is caused by infection with the single-celled parasite Giardia lamblia, also known as Giardia intestinalis or Giardia duodenalis. The parasites live and reproduce in human or animal intestines. Once in the intestines, they attach to the inside of the intestinal wall, where they can disrupt the normal function of the intestines and compete for important nutrients. This leads to the symptoms of giardiasis.