Giardia is a genus of single-celled parasites belonging to the group known as diplomonads. The most common species within this genus is Giardia intestinalis, also known as Giardia lamblia or Giardia duodenalis. Giardia parasites infect the small intestine of humans and other mammals, causing a diarrheal illness known as giardiasis. These parasites have a characteristic teardrop shape and are equipped with two nuclei and multiple flagella, which they use for movement and attachment to the lining of the intestine. Giardiasis is typically transmitted through the ingestion of water or food contaminated with Giardia cysts, which are the dormant, infective stage of the parasite. Once ingested, the cysts release trophozoites, the active form of the parasite, in the small intestine. The trophozoites multiply and attach to the intestinal wall, interfering with nutrient absorption and causing symptoms such as diarrhea, abdominal cramps, bloating, flatulence, and weight loss.

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