Giardia. Since it is generally easily treated with antibiotics, it is not dangerous.
Mildly.... ..But highly unpleasant: diarrhea, nausea, vomiting, weakness, fatigue, loss of appetite.
Anoying > dangerous. As a cause of an acute diarrhea this is more of an anoyance than a threat. Infants have a risk of dehydration but in 3 decades as a pediatrician I don't recall admitting anyone with giardia. Most respond to conservative measures & may not be tested until the symptoms persist past 2 wks. Long term nutritional deficiency from poor nutrient absorbtion can cause wt loss or failure to thrive.