Ryan, could you source that 1913 giardiasis infection study? The only study I could find is reflected in these results:
As you can see, 60% of the men contracted giardiasis with 10 or more cysts. Below the 10 cyst level, only 5 people were tested ONCE, and with a single cyst. There were no tests run at the 2-9 cyst level. There was a 100% infection rate at the 10 cyst level. The sweeping conclusion that there is a 10 cyst minimum required for infection simply makes no sense.
The Welch study you cited I find misleading to the extreme. He studies Giardiasis outbreaks. Backpacker giardiasis will very rarely qualify as an outbreak. In 1991 there were only 80 [giardiasis] outbreaks reported nationwide, or at least studied, out of 34,348 reported cases, out of an estimated 2,500,000 ACTUAL giardiasis cases. His insinuation that because there are few giardiasis outbreaks reported that giardiasis in backpacking must be rare is a major logical fallacy.
As I've said before on this forum, on the PCT in 2010 I wasn't treating my drinking water on the advice of people like Welch. When I went into Mammoth Lakes to be treated for giardiasis, my physician laughed when I told him about those studies such as Welch's, saying he treats backpackers for giardiasis very frequently.
My giardiasis manifesto is here. I welcome any feedback on it.