A flagellated (it has a flagella, it doesn't beat itself with a whip) protozoa.
Water born, food born, and person to person. Many wild animals carry Giardia. Camp, drink the fresh water, get diarrhea. There are day care outbreaks. And swimming pools; kids use pools as a toilet (PubMed).
Watery diarrhea, often prolonged, with bloating, gas and malabsorption.
Metronidazole for seven days.
In failures, quinacrine may work (PubMed).
As a small bowel pathogen, it can be hard to find once it passes to the rectum. It is the reason for O&P x 3.