Giardiasis is infestation of the small intestine with the parasite giardia lamblia.

Sometimes It sometimes occurs in outbreaks particularly associated with contaminated water and can produce episodes of either acute or persistent diarrhoea.

Because the parasite can damage the lining of the small intestine weight loss can sometimes be a feature and if one member of a family is affected then the other members should have their stools tested for the cysts that come from the parasites.