One of their favorite anaerobic places is in a human's small intestine, near where it connects to the large intestine. Giardia lamblia is the creature that most often knocks humans out for a week or two. It lives in two states-one, the trophozoite, looks sort of like a microscopic teardrop. The trophozoite lives in human and animal intestines, where it divides by binary fission and makes thousands of copies of itself that attach to the lining of the small intestine. Humans pick up the disease by swallowing the resting stage of Giardia, the cyst.
Cysts are hardy little tablets that exit an infected animal's body along with feces.
Animals known to carry Giardia include dogs, cats, beavers, muskrats and bears, and possibly Dall sheep and moose. He cited studies on muskrats that showed them to carry even more cysts than beavers that lived nearby. Bemrick also pointed out that infected humans do more than their share of spreading the disease. Three-hundred million cysts may be present in one milligram of human feces, Bemrick wrote, and these cysts can survive for a month in cool water. A science writer motors a canoe, loaded heavy with bags of moose, upstream.