With apologies to those for whom this may be review, let's go back to the question of what the relevant scientific literature says about whether you can get giardiasis from ingesting one cyst and the six papers that were cited above.
* Juranek, Dennis D.: Giardiasis. Centers for Disease Control and Prevention, 1990
This is no longer available on the CDC website. Current references on the CDC website all go back to Rendtorff (see below).
Swartz, Morton N.: Intestinal Protozoan Infections. Scientific American Medicine, 1994
I do not have access to this, but it appears to be a collection of reviews, not original research.
* Kerasote, Ted: Great Outdoors; Drops to Drink. Audubon, July 1986
Kerasote is an author. I don't have access to the article, but it seems reasonable to expect that this was not a publication describing independent experimental investigation of infectivity. If anyone really doubts this, we could ask him.
Backer, Howard D.: Giardiasis: An Elusive Cause of Gastrointestinal Distress. The Physician and Sportsmedicine, Vol. 28 no. 7, July 2000
This is a review, to which I do not have full text access, but as a review, it would not normally be expected to contain new experimental findings, and there is nothing in the abstract to indicate new findings by the author.
FWIW, the abstract concludes by saying ”Prevention for people involved in outdoor sports or recreation and for international travelers includes treatment of all surface water and rigorous personal hygiene.”
* Ortega, Y.R. et al: Giardia: Overview and update. Clinical Infectious Diseases, Vol. 25, 1997
Ortega and Adam is a review article, which I have read. They are commendably circumspect in their interpretation of Rendtorff's findings: “Infections may result from the ingestion of 10 or fewer Giardia cysts."
FWIW, their concluding statements include “Since G. lamblia is frequently found in lakes and streams —
even in remote areas — hikers and backpackers should be warned to boil or ﬁlter water prior to ingestion.”
* Rendtorff, R: The Experimental Transmission of Human Intestinal Protozoan Parasites. American Journal of Hygiene, Vol. 59, 1954
In the absence of evidence to the contrary, it appears that this is the only paper that investigated infectious dose. 0/5 people who got one cyst were infected (subsequently showed cysts in their stools). 2/2 people who got 10 cysts were infected. 6/20 who got 25 cysts were infected.
That's it - the sum total of all the scientific evidence as to whether a low number of cysts can cause infection. Maybe it's my own biases, but when I hear things like "theoretically possible," I'm thinking maybe one in a thousand, or even one in a million. Things that people have looked very hard to find. What does five people tell us? Suppose, for a moment, that the real rate infection for a single cyst is 10% (if I had to bet, I'd bet it's lower, but let's just suppose that for a moment). Could Rendtorff have expected to see an infection among just five people? Nope, he would have had a better chance of getting a hit by spinning a roulette wheel and picking red or black.
He doesn't deal with it explicitly, but Rendtorff understood this. His conclusions had to be based only on what he *had* seen, and he wrote in his conclusions, “The fact that only a few cysts will establish the infection indicates that avenues of infection where it is probable that only a small number of cysts would be transferred may be of importance.”