parasites organisms that derive nourishment and protection from other living organisms known as hosts. they may be transmitted from animals to humans, from humans to humans, or from humans to animals. several parasites have emerged as significant causes of foodborne and waterborne disease. these organisms live and reproduce within the tissues and organs of infected human and animal hosts, and are often excreted in feces. some common parasites are giardia duodenalis, cryptosporidium parvum, cyclospora cayetanensis, toxoplasma gondii, trichinella spiralis, taenia saginata (beef tapeworm), and taenia solium (pork tapeworm).