Giardia infection (giardiasis) is an intestinal infection caused by a microscopic waterborne parasite that's found worldwide, especially in areas with poor sanitation and unsafe water. The parasite is also found in backcountry streams and lakes, as well as in municipal water supplies, swimming pools, whirlpool spas and wells. Giardia is transmitted from host to host by ingesting cysts in contaminated feed or drinking water, through food and person-to-person contact.
Giardia infection is marked by abdominal cramps, bloating, nausea and bouts of watery diarrhea. Giardia infections usually clear up within a few weeks. But you may have intestinal problems long after the parasites are gone. Several drugs are generally effective against giardia parasites, but not everyone responds to them.
Prevention is your best defense.