Giardiasis is very common in most low- and middle income countries, and most persons have been exposed and achieved some degree of immunity as children. In industrialized countries, on the other hand, giardiasis is far less common, and the majority of cases occur in adults either after travel or after water-borne outbreaks.  Both the acute disease and long-term sequels seem to have a different picture after primary infection in adults from that which is known from childhood exposure. A large proportion of adults infected by Giardia develop protracted gastrointestinal symptoms. Some develop a disease with flattening of duodenal villi and symptoms similar to celiac disease, sometimes even with temporary celiac disease immune responses. Many experience some form of subjective food intolerance. Investigations three years after a water-borne outbreak in Norway revealed that more than 40% of adult confirmed cases developed long-term sequels, both in the form of irritable bowel disease (IBS) and chronic fatigue, and the two syndromes overlapped. This has also been observed after other forms of acute gastroenteritis. Prolonged T-cell mediated immune responses have been observed in patients with and without sequels, but with slight differences in symptomatic patients.Similar sequels were also common, but declining in prevalence, six years after the outbreak. Treatment of the initial giardiasis itself was fairly easy using a standardized treatment ladder of up to three different regimens of antiparasitic drugs after initial metronidazole treatment.