This protocol offers diagnosis and standard medical treatment for various parasitic gastrointestinal infections. Gastrointestinal parasites are either worms (helminths) or one-celled animals called protozoans which live in the human intestines. Often, parasitic infections do not cause illness. In these cases, drug treatment is not indicated, because treatment can have adverse side effects. Patients will be examined for their immune responses, correlation between the number of parasites and disease, and other studies.