Here's Mueser (P. 98) "The sample sizes in this study are small, and one is tempted to dismiss the results for this reason. But it should be noted that although the data are from a total of only about 136 people, we are measuring this phenomenon over an extended period of time -- some 15,000 person-days of backpacking during which time the hikers sampled about 1,000 water sources with each person "testing" several hundred sources in the course of a hike. This is no simple compilation of the sickness rate of a few friends hiking over a weekend. The reported cases of sickness may only be 41, but the basic exposure and variety of sources are extensive."
...
"These findings are not the first to hint that how hikers purify their water and how often they do so may or may not be the critical factor in contracting gastrointestinal illness." Mueser then cites a 1993 article in The Journal of Family Practice, based on 180 thru-hikers in 1987-1988, by Byron Crouse and David Josephs.
Table 13-2, P. 95: Methods of Water Purification Used by Hikers
Method : Number of Hikers using Method : Percent of Total
Iodine : 57 : 43
Filtering : 40 : 30
Boiling : 15 : 11
Chlorine : 4 : 3
No Treatment : 18 : 13
Total : 134 : 100
Table 13-3, P. 96: Type of Purification Method vs. Frequency of Gastrointestinal Illness experienced on the trail
Purification Method : Percent of Users who became ill : Giardia only
Iodine : 26 : 6
Filtering : 30 : 9
Boiling : 34 : 3
Chlorine : 75 : 0
No treatment : 27 : 3
Average : 30 : 6
Table 13-5, P. 98: Frequency of Water Purification Versus incidence of Gastrointestinal illness
How Often Purified : Number of Hikers : Number Ill : Percent Ill
Always : 19 : 4 : 21
Usually : 36 : 10 : 28
Sometimes : 56 : 22 : 39
Never : 25 : 5 : 20
Mueser himself finds the results difficult to explain -- though he spends a few pages trying. I won't give it away. Get the book. It's great read.