Giardia (/ dʒiːˈɑːrdiə / or / ˈdʒɑːrdiə /) is a genus of zoonotic anaerobic flagellated protozoan parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates including humans, causing the disease giardiasis. Giardiasis is an infection in your intestines caused by the Giardia parasite. Symptoms include diarrhea and fatigue. Antibiotics can treat it. Key points Giardia is a tiny parasite (germ) that causes diarrhea. Giardia can spread easily from one person to another or through water, food, surfaces, or objects. A healthcare provider can prescribe medicine to treat illness caused by Giardia. Giardia infection, also called giardiasis, is one of the most common causes of disease carried by water in the United States. The parasites can live in streams and lakes, public water supplies, swimming pools, whirlpool spas and wells. The species Giardia duodenalis uses enzymes that break down proteins to attack the villi of the brush border and appears to increase crypt cell proliferation and crypt length of crypt cells existing on the sides of the villi. Giardia is germ (a parasite) that causes an infectious disease (called “giardiasis”) that affects the stomach and bowels. Giardia germ is a common cause of diarrhea in the United States. Giardia infections are caused by the parasite Giardia duodenalis (Giardia for short). A parasite is a type of germ that gets its food from, or at the expense of, its host. Parasites live on or in a host (for example, a person or animal). Giardia is protected by an outer shell called a cyst.