What is the most common intestinal parasite in the U.S.?

Giardiasis is a parasitic infection of the gastrointestinal tract that results from a protozoan organism called Giardia lamblia. Giardiasis, like most parasitic gastrointestinal infections, is transmitted primarily through contaminated water by an oral-fecal route - meaning the organism is ingested orally, the parasitic infection occurs, and infective forms of the parasite are passed in the feces to continue the cycle in other hosts. Giardiasis can also be transmitted by direct contact, one person to another. Giardiasis is probably the most common parasitic intestinal infection in the world, as well as in the United States.