G. lamblia has two stages: the cystic stage and the trophozoite stage. Ingestion of G. lamblia cysts in fecally contaminated water or the fecal-oral transfer of cysts by an infected person results in giardiasis. Giardiasis may be transmitted through sexual contact (direct or indirect fecal-oral contact). When cysts enter the small bowel, they become trophozoites and attach themselves with their sucking disks to the bowel’s epithelial surface. The trophozoites then encyst again, travel down the colon, and are excreted. Unformed feces that pass quickly through the intestine may contain trophozoites as well as cysts.

Giardiasis occurs worldwide but is most common in developing countries and other areas where sanitation and hygiene are poor. In the United States, giardiasis is most common in travelers who have recently returned from endemic areas and in campers who drink unpurified water from contaminated streams. Probably because of frequent hand-to-mouth activity, children are more likely to become infected with G. lamblia than are adults. Hypogammaglobulinemia also appears to predispose people to this disorder.

Those at risk for acquiring giardiasis include employees and children in day-care settings, especially if diaper-age children are present; close contacts, family members, and caregivers of an infected person; people who imbibe contaminated water or ice (including water that has not been boiled, filtered, or disinfected with chemicals); campers or hikers who drink unpurified water or do not practice good handwashing technique; swimmers who swallow contaminated water from lakes, streams, ponds, rivers, and recreational settings; international travelers; and people participating in sexual practices that expose them to feces, such as anal intercourse.

The mucosal destruction caused by the protozoa decreases food transit time through the small intestine and results in malabsorption. Other complications include dehydration and lactose intolerance. Giardiasis can be life-threatening in patients with hypogammaglobulinemia. It can also complicate conditions such as cystic fibrosis.