Intestinal parasites are generally divided into two main categories: Protozoa and Helminths.

Protozoa: These are microscopic, single-celled organisms. Common protozoan species that cause intestinal infections in humans include Giardia lamblia, Entamoeba histolytica, Cryptosporidium, and Cyclospora cayetanensis. These parasites typically spread through contaminated food, water, soil, or surfaces soiled with human and animal feces. For instance, they can be found in poorly maintained spa waters or swimming pools. Some protozoa form resilient cysts that can survive outside the host for months. Ingesting these microscopic cysts or parasites results in infection.

Helminths: These are multicellular intestinal worms visible to the naked eye. Helminths are generally classified as flatworms (trematodes and cestodes) and roundworms (nematodes). The flatworm group includes tapeworms, while roundworms encompass many species, including pinworms. Specifically, pinworms (Enterobius vermicularis) are one of the most common intestinal helminth infections in humans, particularly in children.