Giardia

Giardia lamblia is a single-celled, flagellated protist parasite that infects the small intestine of humans and other animals, causing the diarrheal illness giardiasis, one of the most common intestinal parasitic infections worldwide. It spreads through water or food contaminated with a hardy, dormant cyst stage that can survive outside a host for extended periods, which is why infection is strongly associated with untreated drinking water, and it remains a leading cause of waterborne outbreaks of gastrointestinal illness even in regions with otherwise modern sanitation.