You are asking an impossible question. I suspect that the majority of people who pick up infections from backcountry water sources never correlate the illness or never get sick at all... it's estimated 50% of the folks who have been exposed to giardia never develop symptoms.

I for one will not play roulette with my health, because I personally have spoken to well over a dozen people now who have had confirmed giardia cases with some pretty nasty symptoms. One guy's dog nearly died of it. All have been in California, as that's where I hike.

You can be sure that sources in areas heavily trampled by livestock (whether grazing cattle or pack stock) are a high risk for contaminated water sources.

Iodine is a pretty poor method of treatment - it's not at all effective against crypto and does only moderately well against giardia.

There have been some studies in national parks here in California (now somewhat dated) that verify the presence of e coli in the more heavily used areas. Merced and Tuolumne Rivers were among them. You can google terms liked water, bacteria, yosemite, mountains... it won't give you the numbers you are looking for.

You also need to be aware of mining activity or other industrial activity upstream of where you are going. The filters and purifiers do nothing against chemical or mineral contaminants. Some springs in some areas have heavy mineral loads that will give you bad gastrointestinal issues if you drink them....

Salmonella, btw, is not one of the likely infections you'll find in water sources.