Giardiasis (also called giardia) is caused by infection with the single-celled parasite Giardia lamblia, also known as Giardia intestinalis or Giardia duodenalis.

The parasites live and reproduce in human or animal intestines. When they are in the intestines, the parasites attach to the inside of the intestinal wall, where they can disrupt the normal function of the intestines and compete for important nutrients. This leads to the symptoms of giardiasis.

• Disruption of the intestinal surface can reduce the body's ability to absorb nutrients from food passing through the intestines. If not absorbed, nutrients remain in the intestinal tract, where they may contribute to the diarrhea, cramps, and gas associated with giardiasis.
• The parasite may cause food to move more quickly through the intestines, which can contribute to diarrhea.

The stools (feces) of infected animals and humans contain the parasite. Infection with Giardia lamblia occurs in two ways:

• Direct hand-to-mouth transfer of the parasite from the stool of an infected person. This may result from poor personal hygiene, such as not washing hands. The parasite can also be directly transmitted during oral-anal or oral-genital contact with an infected person.
• Indirect transfer of the parasite by drinking water or eating food that has come in contact with feces containing the parasite.

The incubation period averages 7 to 10 days but may range from 3 to 25 days or longer. You can spread giardiasis as long as the parasite is in your feces, which can last for months. Not all infected people develop symptoms, but a person who does not have symptoms can still spread the infection.