Giardia was so common in the absence of symptoms that in those days it was not
considered necessarily a pathogen.

Single-celled protozoan parasite. Pear- or beet-shaped, the cells have
two nuclei and eight flagella, and attach to human intestinal mucous
membrane with a sucking organ. They cause the disease giardiasis.
Generally spread by ingesting traces of human feces containing the
parasite, giardiasis is most common among children in close contact
with other children, but also occurs among adults. Diarrhea, pain, and
distension of the stomach may occur. It is common wherever there is
contamination of surface or domestic water in lakes, rivers, and
reservoirs, and is a major cause of diarrhea worldwide. Beaver feces
are often responsible for giardiasis among campers who take water
from lakes and rivers.