The water risks that you encounter depend a lot on where you pick it up. For example, if you are downstream from a village in a third-world country that has no facilities, you stand a chance of picking up a viral or bacterial problem. [I saw a lot of that in Nepal.] You can knock out a lot of viral or bacterial risk with boiling or chlorine/iodine treatment. Still, you want to look around to avoid some decomposing animal carcass in the water just upstream from you.
If you are picking up raw water from a still lake at 7,000-9,000' elevation in California, you stand a little chance of viral or bacterial problems, but Giardia is more likely, but still nowhere near a certainty. The risk increases if you are downstream from cattle or other domestic animals. Note that Giardia exists in two forms. One is the cyst, and it is tiny, maybe only 2-3 microns, and it is a hard little egg. Filters, chlorine/iodine, boiling, or maybe UV will work. The other form is the adult, and it is much larger, maybe 25-35 microns, so the simplest filter will knock it out.
If you are picking up cold snowmelt at 13,000' elevation in California, you stand little chance of any risk, but that does not mean absolutely zero risk. If there is a big snowfield with no muddy little mammal tracks, then you are getting much closer to zero risk. I've seen lots of muddy little mammal tracks above 13,000' in a few places like Mount Whitney.
Chemical risks are easier to predict. In the U.S., most serious chemical risks will tie to mining residue or to the actual ores being mined. Many mines are marked on maps and controlled by the feds, so they are not hard to avoid. I would not drink the water flowing out of a mine or especially from the area of a ore mill, but a mile upstream from it the water is probably great. I don't operate around many mines, so I don't worry about this. You want to understand the term "heavy metal."
Due to where I operate in California, I use a 2 micron gravity filter routinely. Then I also have some chlorine bleach that I add if I am paranoid that day. Although it is possible that I got some strange water, I have not had any such symptoms in forty years now, since I was in the Army and got water in the field.