Final answer:
Giardia causes diarrhea by attaching to the lining of the small intestine and preventing the host from absorbing nutrients. They also block the absorption of fats. The movement of their flagella creates an area of lower pressure that promotes their adhesion to intestinal epithelial cells.
Step-by-step explanation:
Giardia is a flagellate protozoa that causes giardiasis, a type of gastroenteritis. When ingested through contaminated food or water, the parasites attach to the lining of the small intestine. They prevent the host from fully absorbing nutrients and also cause diarrhea. The attachment is facilitated by a large adhesive disk comprised of microtubules, and the movement of Giardia's flagella creates an area of lower pressure that promotes adhesion to the intestinal epithelial cells. Giardia also blocks the absorption of nutrients, including fats.