The precis of this protocol is to allow the evaluation, treatment and study of patients with a variety of gastrointestinal parasites. This protocol primarily allows evaluation and treatment of patients with any intestinal parasite that requires a medical evaluation. The treatment and evaluation consists of standard of care. Research aspects include the collection and study of different parasite populations, analysis of the immune responses of the host, a correlation between parasite burden in the host and disease. These patients also serve as a source of reagents such as feces, white blood cells, and serum. The off-label use of FDA approved drugs is employed to treat symptomatic giardiasis in patients who cannot be cured otherwise and to determine empirically which regimens are effective and safe.